The normal way to tell if a trap will not be active any more is that the trap does not have that 'sprung' look to it like in a new fully opened trap. After 2 or 3 'meals', the teeth are not poised in the same way (sometimes pointing vertically up still in digestion mode even when it has opened up) and/or the main part of the trap is still concave.
Need useless traps be pruned off immediately? If not immediately, should they be pruned off, ever? I tend to think that the energy required to maintain a useless trap would be better spent on creating new, healthy traps. Or does the photosynthesis benefit of leaving the trap outweigh the energy allocation drawback?
i do know that part. yes leave the trap be. like you said it will act like a normal leaf. so i think the outcome of having a useless trap would be benifical for the entire plant. even though the trap wont be able to catch prey, it will still make food.