How do you find the hole in a legweight/sandbag tear?

Question:My sandbag thing i use for exercising (tie to my legs) has a tiny hole in it which makes it spill the sand slowly. Thing is i cant find it by using any conventional method, cant even see where it spills from (the sand thing is black).

How do i find it if anyone knows?

Answers:
Unless you have tried this hold it closely over a white piece of paper and shake it around to see if you can narrow this hole down.