How can I tell if there is a water leak in my wall?

Recently, I have been hearing what sounds like dripping in one of the walls in my downstairs bathroom. However, it seems like the leak is only heard when I flush the upstairs toilet. My father seems to think that the sound I hear may be normal. However, I feel like it is a recent development which would mean its not normal. I only hear the leak while the toilet is refilling and when it quits, the leaking sound goes away. So, I don't know if that is strange or what. I don't want to open the wall if there is no leak but at the same time I don't want to wait until there is alot of damage to fix the leak. I just wanted to know if there was a way for me to know definitively that there is or isn't a leak before I hire someone to tear up my wall.