I know there are probably all kinds of ways to find out, but here are a couple ways to know:
1. IQ tests are NOT the best indicators for prodigy-like giftedness. What if your child knows how to fix anything about your car, or how to paint glorious scenes from from nature, or can almost talk to your pets, or it least it seems like it since he knows what the pet wants, if it's hurt, and where it is hurt? These talents cannot be measured by IQ tests. So you have to find some sort of subjective test.
2. If your child does something that dazzles your entire community, and it just happen to be a skill of some sort, you are probably safe to say your child is at least gifted in some sort of way.
3. A good way to know if your child is a prodigy is to talk to an expert in the field. If your child is "gifted" at painting, get an expert's opinion. If your child is "gifted" at solving complex mathematical equations, again, get an expert's opinion. Now this may be harder, or even more expensive, be it is very effective.
I hope this helped:)