Sigh...plumbing leak

My neighbor below me has water spots on his ceiling. He finally let me know...2 MONTHS LATER!! Of course, now that I've finished my bathroom and I've patched all my holes and everything...NOW he lets me know. Argh. I didn't do anything to the plumbing so I have no clue where the leak could be from. However, when I had the walls open, I did have some water spills. I'm wondering if it is actively leaking or if it was an old spill I did that stained. I asked him to check if it's always dry or if it is actively moist. It could very well be some of the spills that I had. I don't see any leaks behind my toilet, (I never removed the stool when I did work). I don't notice any wet walls or anything on my end (not that I always would) but does anyone have any pointers for how to figure this out before I have to spend money I don't have (as I'm unemployed) on a plumber? Wish he would have said something when my walls were actually open.