No special tool that I am aware of. Since you don't know what is behind the wall you have two choices that I can see.
1. You can take your chances and put enough pressure on it to get the pipe to break loose.
2. You can carefully open the wall above the tile around the elbow to see what you have.
If you try the first option you may break something and have to open the wall anyway. If you go for the second you will definetly have to fix the wall but not the plumbing because you can get to and support the threaded fitting the shower elbow is screwed into. How lucky do you feel?