I know if someone's troops are under 20% then they're too weak But other then that, what would make someone too weak to attack?
People who will always be too weak for you are those whose total power (from buildings plus ally bonuses) is significantly less than your total power, I think it's by a factor of like 4.2 times if I'm remembering right, somewhere around there anyway. A way to test if they're too weak for you because of total power or if they're temporarily too weak because of their troop level or them being at $0 is to scout them. If you cannot scout them at all, it's a stats thing. If you can scout them, it means their troops are too low or they have $0 out.