Leave a party if you see You see people going to the bathroom in groups of three or more

These are my suggestions born of my own and my friends experiences, that said it describes almost every party I have ever been to. Be careful, and watch your drinks, do not accept drinks for strangers, and do not drink from drinks you have put down. Know that you are responsible for your own safety.

You see people going to the bathroom in groups of three or more
This is pretty sure indicator that large amounts of drugs are being used/sold.

A bright shining light keeps passing over the windows
This means the ghetto bird is out and the party is probably already surrounded by cops.

You hear fireworks going off
This is really bad sign, since fireworks are banned in most states, this generally means you are getting shot at, leave quickly!

You think you are having a highschool flashback.
This means the party has been overrun by underaged drinkers, and you should leave quickly to avoid arrest.

You were invited to a party but find only a few guys there
(for girls, or possibly boys depending on the situation)

This generally means your hosts have plans for you, that you may not like. Do Not Enter, Do Not Pass Go, Do Not Collect 200 Dollars. Run Away!