Exactly, just take a fuel "sample". Enough to fill a qt jar half way. Take a big whiff, if it smells like gas and not varnish and doesn't look like varnish, you're good. No need to drain the tank if the gas is good. Since it's full, I'm guessing it's just fine.... I didn't look back in this thread, but if you haven't bought it yet, have them fire it up again while you're present and let it run for a while....
Ok, I see you bought it. Run it on muffs in your backyard for a while and see how she goes. After it warms up, run it at a little higher rpm and see if it runs smooth.
Maybe whenever you dump it in the lake stay relatively close to the landing. If all still goes well, take off and have fun....