-When they ask for your current/previous salary in a job interview, tell them that you don't find that relevant or that you don't want to tell. If they insist on you telling your salary, GTFO

- When they are overenthusiasticly telling about all the latest technologies they're using without staying one word about legacy projects, GTFO. It's a trap.

- If you walk trough the developer room(s) and everybody is extremely focused and just programming like a zombie, GTFO.

- If they cannot tell you one single downside of the company, it's probably too good to be true.

That's about everything I can think of at the moment