It becomes clear very quickly. They treat the free crew like ****. And you need to develop a good bullshit meter for bad projects. You can ask to read the script first before getting involved. And always ask a lot of questions.
Follow your guts dude. If it makes you FEEL GOOD doing it, DO IT. If you don't feel comfortable (whatever the reason), walk away and don't look back! (money or no money! ! ! )