Is it really just trial and error or is there certain signs that tell if a wrong warp is possible? Or maybe looking at the code?

It is not something that is gonna be in every game. It has to do with executing certain actions to mess with the games memory. If anymore are found it will prob just be on old nintendo era games. (Because limited memory)

If you were to find a wrong warp you would likely have to be looking at the code for the game. You will need to be finding what bytes the game calls first when you originally warp, and then you will need to see if you can change those bytes to something else.