Well for one, never accept payments from unverified accounts. In my experience, 99% of them are fake.
Other than that, scammers are pretty lame and usually keep usually the same IP, email address or writing style. It is pretty easy to detect one. Also an anti-fraud system in place goes a long way.
In case all this fails, just put all the data in paypal and let them decide. You most certainly do not have any other option.