Do you know if they report them to your school?
That may sound strange, but one of the reasons I started this website was because every other scholarship search website out there returned nothing but the "no essay contest" results whenever my daughters searched. It was frustrating. I even signed up on the sites and tried different stats to see if I could get more of a response and had no luck. I think some sites have improved and some newer sites are more transparent with their listings, but my youngest graduates in May, so those contests don't help now.
That isn't a very helpful answer...so here is the helpful bit. I think all of the contests are legit, but I also think the odds of winning are pretty slim. Still, if you put each deadline on your calendar and make sure to enter each time, what's the harm?