Topend rebuilds aren't determined by miles or hours, but by compression. Some riders on small displacement two-strokes only get 5 or so hours out of their piston and rings while someone who putts around on a large-bore bike can go over 100 hours. It largely depends on riding style. With 800 miles you could have anywhere between 20 and 100 hours on the bike.
If the bike feels good, it probably is. But to be sure, go to your local autoparts store and purchase an inexpensive compression tester.
Keep in mind however, just because the piston and rings are good, that doesn’t mean that the crank hasn’t run-out. This is more of an issue for someone who has performed several topends.