A good leather jacket has weight to it before you've even got an arm in the sleeve, that's the first thing. You pick it up off the hanger and it doesn't flop, it hangs with a bit of body, and the leather itself feels more like a glove that's been broken in by someone else already rather than stiff cardboard pretending to be hide. Zips should feel solid under your thumb, not like they'll jam the first time you're in a rush. If it feels light and a bit papery, put it back, that's a jacket that's going to crease in all the wrong places by October.
This is an autumn piece more than anything, and a early spring one too, the bit of the year when a coat is too much and a cardigan does nothing against the wind. It struggles in proper summer heat, obviously, and it's not made for standing in a downpour either, though a light shower won't hurt it, it'll just need a wipe and a chance to dry flat rather than on a radiator. Over the first few months it softens at the elbows and across the back from actual wear, which is the whole point, a leather jacket that still looks showroom new after a winter hasn't been worn properly. The collar and cuffs take the most punishment, so keep it away from anything greasy, don't shove it in a cupboard folded in half for months, and give it a proper leather conditioner a couple of times a year rather than whatever's under the sink. Scuffs and small marks aren't a disaster, they're basically the jacket keeping a diary.
If you've never owned one, start plain and start black, a classic biker shape with a waist that sits where your actual waist is rather than somewhere up near your ribs. Skip the cropped version first time round, it's a lovely third jacket and a difficult first one, because it only works with high waisted everything and you'll be annoyed by how little it covers on a proper cold day.
The best detail is the sound it makes settling onto your shoulders when you put it on, a quiet little creak that means it's actually leather and actually yours.