Pinkbike's trailforks app is my go to research tool. I'm starting to trust the popularity rating more than the actual rating of the trail. Strava heatmap is also a great way to find some hidden gems.
There are tons of GPS based websites and apps that can help you find new and interesting rides in your neighborhood! RidewithGPS, Komoot, and Strava are a few that I've used. Next time you're in your local REI, ask some of the green vests what their favorite mountain bike rides are, maybe you'll find some new gems!
@jfkand @REI-LizH are right with their suggestions. One of the best techniques I have used is to go to your local shop and buy a trail map and describe your skill level and desired ride time/distance to an employee and have them highlight a few good routes for you. People are usually willing to share at least some well known trails and you get a map to remember your adventure and plan the next.
Sometimes you can look up trailguide books or find them at used stores. They might be a little out of date but can often get you in the right direction. Don't forget to check your local library first!