From my experiences, the professors that you aren't able to figure out what their political leanings are, tend to be either moderate or right leaning. The liberal profs stand at the front of the class spouting out their views and MIGHT add a few facts on the subject they're supposed to be teaching.
Yep. Thats usually how it works. Liberal professors often shout out their conservative counterparts, so they tend to keep their own views quiet or hard to discover. They tend to just teaching the subject matter, instead of their own viewpoint on the subject matter as well.