I thought everyone knew these, but found out the other day that some do not. When trap is combined with lure, the direction of the trap (at least for some of them) indicates wether or not the lure is going to work. Of course knowing this isn't really useful. It might be good to let you know when to have a unite handy with ubers. It's kind of fun to beat the punch with a nice shot or rats. I'm not sure if there are also ways to tell with banana peel, marbles or TNT.

If a rake falls angled left, it will fail. If it falls angled right, it will work.
Non working rake:


If a sand trap falls with the notch towards the cogs, it will fail. The notch towards the toons will work.
Non working quicksand:

Working quicksand:


If a trap door lands with the hinges on the right, the lure will work. Hinges on the left will not.
Working trap door:


Note: most people disagree on the rake thing. Angle doesn't seem to mean anything.
With banana peel, marbles and TNT, if you see it there on the floor before it's thrown, it will work. If not, it won't. This only applies in buildings - street, boss or HQ battles it doesn't happen.