For me, I have a small process:
1. Watch the trailer. So I can get an idea what it's about.
2. Then I check IMDB. I like to see the actors involved, what they have done recently. Check out the writes/directors/producers, what they have done recently.
3. Then I'll check the IMDB topics about the movie. Most are trash and spam, so you gotta know what to click and what to skip over. But I'll read other people's opinions.
4. Some instinct, sometimes I'll watch a movie even if everything above doesn't look very good. Sometimes I enjoy it, sometimes I can't finish a film.
That's about it for me.