Never pay an upfront fee or incur upfront expenses.
Legitimate employers do not charge you a fee in advance of work nor do they expect you to incur significant expenses before getting paid. For instance, if you are signed by a legitimate talent agency, the agent earns nothing until he/she has found you a job. The agent receives a commission when you get paid for your work. Likewise, a legitimate employer does not expect you to pay out-of-pocket travel expenses to get to a job site unless you have a written contract that guarantees work when you arrive. Please note: this also includes an STD test. You should not pay upfront for such a test. Beware in particular of anyone asking you to wire funds by Western Union to pay upfront for an AIM test.