If it's your major, you pretty quickly figure out which professors are compatible with your own political opinions, which professors are truly open minded (i.e.: will be fair with you even if they disagree with you) and which ones should be avoided at all costs.

There are some truly bad ones, unfortunately, and your comment about TAs brought me right back. There's nothing like the petty tyrannies of a little clique of TAs operating with nearly complete impunity within their professor's fiefdom. The malignancy with which some of the Poli Sci TAs would punish conservative students—the absolute, hypocritical self-certainty with which they carried themselves while asserting that they were the openminded ones—was something to behold.