Here are a few suggestions.

Go to the place at around 10 pm on a Saturday night. Most places are quiet during the day; you get an idea of what the area is like after dark.

Bring a phone charger and test every outlet. Check every faucet and see if the water gets hot.

Step into the shower or lay down in the tub.

Be sure you can bring in the furniture you already have. Some places have weird, narrow passages.

Make sure that there's a grocery in walking distance.

edit = I forgot something. A lot of places these days are illegal conversions; houses designed for one family that now have several apartments.

If you place is a walkup, make sure that you can get out in case of a fire.

I pay attention to the condition/cleanliness of the hallways and stairs in the building. If the owner can't be bothered to maintain any of that then you already know the apartment itself is going to be a mess as soon as something goes wrong.

A bit harder to judge but if it looks like other tenants/random people make a habit of hanging out in the hallways/stairwells then that's a massive red flag. One time I went to see an apartment and a guy inside the building on one of the stair landings was chilling out smoking a massive cigar next to the window.. I knew right away that building is always going to have cigar smoke.

This one might sound silly but I have a habit of testing the water in the bathroom and kitchen. That tells me what the water pressure is like as well has how well the hot water is working.

Maybe a bit nit picky but I usually bring a tape measure and do some quick room measurements to figure out if furniture is going to fit & whatnot.

Oh and like the other comment said it definitely helps if you know what the area and the building surroundings are like at night.

Shoddy or delayed repairs.

Before you rent, open all the cupboards and drawers, look for tell tale signs of pests. Feces or dead bodies, hairs.

The longer the list of what you're responsible for the less likely you'll get your security deposit back.

Not letting you take pictures while inspecting an apartment.

Check all the drains and faucets, flush the toilet. Make sure there's good water pressure and drains are cleared.

if at all possible, check to see how old the buildings/property is. My dad noticed that my first apartment was from at least the 1960s or 1970s because of how high the light switches were (it was before regulations that required those to be handicap accessable). Also when I started putting furniture in, I put a shelf against one wall near a hallway and the wall was slanting inward, which I wouldn't have noticed if not for the shelf

Don't let them bait and switch you with an apartment.They might show you an apartment that's in really good shape and act like that's the one you're gonna get, and then they give you a different one that is not in good shape, so make sure that the apartment you're looking at is the exact apartment that you'll be getting.

Also, don't skip this one, talk to the neighbors. I know it's awkward and you're not going to want to do it, but it's really the best way to get information. They will tell you how things actually are.

If you live in a place with proper winter, look at how well the sidewalks around the property are maintained after snow or a bad freeze thaw cycle or the like. It's a good indication if property management is on top of their shit or if problems will just be ignored until the city gets involved.

If you see any empty beer bottles on nearby doorsteps, know you're not gonna get any sleep on the weekend if you move in.

The no-bite management is so true. At first "cool they're chill", but when our baby's bedroom was getting smoked with weed coming from the downstairs through the a/c vent every single day and the neighbor downstairs ignored my requests to smoke on the balcony rather than in the room below my baby, all I got from management was "so what? I smoke too".

Do you really want your landlord to play judge, jury and executioner?

That can easily backfire against you

is a police report

If they're in contravention of city by-laws, then that's what's needed. Ofc they might be saying that just to scare you or whatever, but having an actual police report about the issue really lights a fire under their ass. I did this with one neighbour that would always start blasting music at 2am, like it was his own personal nightclub. It took months, multiple phone calls, and visits from the police but when I finally filed that police report they were evicted a couple months later. Don't be shy about calling the police to deal with those issues (unless you're American) as that's one of the services they provide (unless you're American).

One tip I can give is see how many people have ac units in their windows. If there's a lot, then the building doesn't have good insulation. Which is an issue one can deal with (using window mounted ac units), but the rent should match that.

When you have seen the apartment e.g. during daytime during a weekday, have a look at the area in the evenings and on weekends.

There was a story in the news where someone had rented a flat close to the city center, only to learn that two roads over is a amusement mile of national renown. Which was rather quiet during the visit, but lout and filled with drunkards and criminals every evening and weekend.

Don't rent apartments with cardboard walls.

If you can, talk to the previous tenant. They might be moving out because of problems with the apartment. Whether that's an annoying neighbor or a mold problem, you probably don't want that either.

If something seems too good to be true, it probably is. I was shopping for a 1br apartment, and I found a listing in a good neighborhood. I was suspicious before I even saw the place because I knew the neighborhood was all duplexes with 3 bedrooms. Normally a duplex in that neighborhood was $1800 - $2100. I went to see it and sure enough it's a duplex where each unit was 3br apartment that has been converted into 3, one-bedroom apartments. They were asking $800 for a 1BR you couldn't event fit a twin bed into the bedroom. I spent 5 minutes looking around and told them no thanks straight away. They called me back 1 month later asking if I would consider for $600 a month. I was like I wouldn't live there if you paid me.

Pets - From experience, people are AWFUL with their pets. Mostly dogs, I've never seen anything go wrong with cats, unless the owners don't care enough to let them run around until they're kidnapped or ran over. But dogs, they just let them go and go with the barking. Not to mention the dog shit on the ground they refuse to pick up.

What a complicated way to say that you dislike dogs/pets.

Any broken glass in parking areas, no age diversity, dirty smelly unlit damaged hallways at all, smell of smoking in the buildings at all, not up to fire code in any way, the free parking around the area is totally full at any time of day, decorations of decks or windows banned, they don't start off with you previewing THE EXACT unit you'll be renting, they ever walk you through what seems to be a preview unit.

If you're there when the mail is being delivered, talk to the mail person. They're there every day and can have some good insights you won't find anywhere else. For instance, once when I did this he told me he gets a lot of mail forwarding notices, and more often than not the boxes are staying empty after that. I didn't know why that place had a lot of people leaving, but it was enough to make me keep looking.

The fact that it’s an apartment.

If you use electric heaters and have gas, your a fucking idiot costing everyone money.

Not sure what makes sense to ask but my personal experience:

Find out if any of the neighbours smoke and if the smoke gets into your appartment as well.