How to detect patch on drywall
You can try a magnet.  They will have to secure any substantial patch with a nail or screw.  But really there is no exact way to know what was the patch and what was existing drywall, because the existing drywall will also have screws/nails around it.  

So either the patching will be in a unique shape and you can find it or it isn't and you need to take pictures of the wall before they patch.

Real world I seriously doubt this will be patched absolutely perfect.  Not only would they have to do a superb job with the drywall and mud but they would also have to prime it and paint it feathered in with the existing area.  So in reality a high powered light and a good angle on the wall and you shouldn't have a problem.