According to a flier issued by the FBI on how to spot suspicious activity, if you see someone paying for small items like a coffee with cash, it could be a sign that they are up to no good!
Other such activities which could be deemed suspicious are:
If you try to shield your computer screen from others, (yeah come look at my password).
Carry more than one mobile phone.
Use VOIP or in-game chatting.
Oh the joy! The full flier can be found here: http://info.publicintelligence.net/FBI-SuspiciousActivity/Internet_Cafe.pdf