How to I check a used iMac before buying?

[mac121269]

Hey,
I have been looking to buy a used iMac, for I can not afford a new one right now. I want something reliable, that is why I am looking at iMacs. I have been looking at the 21.5" screens and want at least 500G HD. Other than that, I do not know what I want. It is for home use; mainly email, internet, and photo. Maybe watch movie here and there.

I plan to look at one I found on craig's list this weekend, yet I don't know what to look for when inspecting it.

Here is what is advertised:
late 09 imac. It is perfect in every way. Condition is excellent and functionally perfect as well. Currently running OSX Mountain Lion.

Here are some specs:
21.5"
Intel Core 2 Duo
3.06GHz
500GB HDD
Upgraded to 12GB ram using Crucial Memory
Apple wireless keyboard
Apple Mighty Mouse

price is $600

Any suggestions and/or advise?

THanks!