How do you tell im overwatering?

It's just a guess, really. I know that a large proportion of cacti growers do do that, but that's not the reason. I don't know this particular species though, so like someone said, maybe it's supposed to be like that. But I just think they look a bit "chunky" in the sense of being brimming with water. And often overwatering causes droopiness and a kind of darkish hue in other plants.

Overwatering is so easy because it's really hard to overestimate the water needed. Like I said, you can (and indeed need to) put the cutting of the cacti in a dry place, so that the cut will heal over, and then it can be replanted, and the portion which will be planted ie not in light but in (occasional) moisture, it will start developing roots.

But like, again, am no cacti professional. I got one from a friend like 20 years ago. It almost died at one point but then I had a few cuttings that I thought were maybe too dry to make it idk but seems they're getting some color back. The small ones.

The big one is Peruvian Torch or at least something related, echinopsis afaik. It does have mescaline, that I know. This is my biggest one yet, I've had like 5. Started with a cutting the size of my thumb. I nourished it and got a cutting from it (as I was just holding it for a friend) and have since taken like 3-4 largeish cuttings from the plant. And now it's that huge, I think I'll do a few cuttings soon, because I'm gonna run out of vertical room otherwise. Well not in the tent but I'd have to raise the light and then raise the other plants closer to the light if I want them to have as much light. Anyway.

Like my small cacti are lying down as well but that's mostly because they haven't made proper roots and then grown, I think they will straighten them out if/when they develop. But at least the big one is doing wonderfully. And drinks quite a lot too, actually. I'm gonna take some cuttings to shorten it, hoping it will increase thickness. And from this for instance you can see the different hues on the large one. The top being a bit paler might be even light burn actually, but also, can indicate sort of "hunger". Plants get paler as they have less nitrogen to use and overt nitrogen shows up as dark heavy leaves and even burn. But yeah even though I haven't grown these plants myself and know round cacti exist and whatnot, I don't know, they just seem "plump" and it's a smallish cup.

Also if you replant them, bro-tip, you could get those "cactus soil mix" from a store, but I've often found) (at least where I live) that they don't have nearly enough actually sandy material there. So one of those cacti soil bags and then get yourself like a small 3-5l bag of perlite. And then mix it 5050 with the "cactus soil", and that's usually pretty good. I tried for less perlite with the big one this time, but I'm not watering too heavy. It sits in cocoa fiber mixed with some perlite and vermiculite. I never really fully water that pot. Or if I do then I leave it until it dries mostly. Lifting the pot method is a great way, btw, to appraise how much water the pot/plant has. Just lift the pot they're in and by it's weight you can tell if it's really wet or sand dry, or something in between. Works for most plants and pots Imo.

I'm rambling a bit, took an ambien a while ago, going for a nap.