Recognizing a trap on a chest is a Perception roll - look for the die to appear over a character's head when approaching the chest. If you fail, you don't get the disarm option in the right-click menu.
That chest will always do minor fire damage, even if you pick the lock and disarm the trap. Since it is near oil, it will technically do major fire damage. If you find the key, it will open without any damage. Key location:
There is a body on the back side of the island in the waterway. You can jump to the top of the island from the lower path.
I strongly suspect both of those are deliberate.
There is one other chest that will do automatic damage even if you pick the lock and it doesn't appear to allow disarming the trap. On the other hand, the description makes it clear there is a trap, so you can prepare for it. Location:
The hidden basement under the smith's house in Blighted Village.
Safe options:
Get the loot from the shelf above first; otherwise, the chest destroys the ladder. Any damage triggers the trap; the chest is tough enough to survive minor damage. So stand back and shoot/spell/throw something at it.
I personally consider both chests ‘fair gameplay’ - there are warnings and options, even if they aren't obvious.
