How do I tell if these are legitimate?

I have been seeing a bunch of websites around lately that claim to have art contests, and this is really interesting to me as an artist. However, I am wondering how to tell if these contests are legitimate. I don’t want to be taken in by any scams. I would only be interested in entering the free contests anyways, but is there something I should look out for, or are these on the up and up? I would really love to find out more about which of these sites I should try to submit my artwork to for consideration if anyone has any experience with these.