If buying used a Honda generator...what should I check?

I would make sure it's not leaking oil, no parts are missing/broken and everything looks right.
Check the oil and make sure it starts easily. It should run quiet and not be smoking.
Take a Volt Meter and check the output. Then put it under a load and make sure it continues to run properly.

If the seller refuses to let you run or voltage/load test the Honda, walk away.