I have a spreadsheet.

Each month I check the balances of all my accounts (eg: savings, checking, vanguard). If anything looks unusual, I go investigate and make a note of what happened. (Eg: bought expensive boots here, moved money from this account to that account). If I was doing that a lot, I'd probably change something about my routine.

I used to export all the transactions from the bank websites and import them, but I found I wasn't really using that data.

It helps that I'm naturally very frugal, and don't have a lot of expenses.