You may be able to find a guidebook to mountain biking in your area; they often rate technical and aerobic difficulty separately, and give detailed descriptions of the route that may help you decide if it sounds good or not.

One problem is that a trail can change from when it was first rated.

There are a lot of trail sites out there, but their content is sometimes pay-only or just plain spotty.

Here are a couple --

http://www.singletrackrides.com/

http://www.singletracks.com/

I'm not necessarily endorsing them; they're just a couple I've looked at recently.