I had to evaluate a used generator recently and I used a Kill-a-Watt device to measure output frequency, voltage and current under load. I took some portable heaters with me so I could create a load (this one was 5000 watt)
Kill A Watt from iKitchen.com
Obviously, you'd also want to look at smoke from the exhaust and other tell tale signs of hard usage. I've found that home owners buy these for emergencies and the worst abuse comes from sitting with old gas rather than wear.
If the generator doesn't have an hour meter, you can't tell directly how many hours it has been run - need to use other cues.