There are numerous tests/benchmarks you can compare with, but I would focus on:
1. It working (and booting to OS)
2. A legit, activated OS
3. The hardware being what you expect to see (as listed)
4. Reasonable temps at idle/load (HWMonitor, Speccy or MSI Afterburner for temps, Prime95 and Heaven benchmarks for stress testing)
You could run Furmark benchmarks to ensure you're getting performance relative to others with the same setup (keeping whether a setup is OC'd or not in mind for comparison).
As far as not getting "screwed", trust your gut. If something feels 'off',it probably is.
Ask questions:
How long they've owned it?
You want to hear a relatively decent amount of time, preferrably first owner. You don't want to hear X weeks/days etc.
Why are they selling?
You'll want to hear a reasonable reason (upgrading - and they'll probably tell you all about it), need the money etc.
Was it overclocked? Some people will shy away after hearing "yes", but if it's somebody knowledgable, then get the info:
What speeds/voltages/temps etc
I doubt this is the case, but don't necessarily be scared off by a little dust or overheating (within reason), as lots of people don't take care of their setup. You can have a nice little rig for fairly cheap if you use that to your advantage (and clean it yourself, of course). I bought a first gen i7, 8GB DDR3 and a decent GPU for $40 CAD because it was pretty gross and nobody was interested
What are the specs, out of curiosity? and what do you anticipate paying for it?