I usually interview them. Asking a lot of questions during the interview. The bad employers/HR often get frustrated because they see interviews as some wacky control/power thing where they expect your questions only at the end. In reality I see it as a way to find out their views on what they see as just one of many employment situations. After all, they're trying to sell their workplace as being good to operate in. If they're unwilling to answer basic questions about it then they're suspicious.

This has become a good filter.