Unless you have the paperwork from the previous owner indicating so, it usually isn't possible to know without pulling the heads off to check - and even then it could be a bit of guesswork depending on how long ago it was done. One thing for certain, these bikes come originally with 7mm diameter valve stems. Sometimes when people have done a valve job and replaced the seats, they've also had the valve guides, valves, and keepers replaced with 8mm diameter valves - which is what the larger R80 and R100 bikes use. This requires a bit more machining work to accomodate the larger guides and valves.
If your valve stems are 8mm diameter, that would probably be a semi-reasonable indication that someone had replaced them, and if they did replace them they likely did the valve seats as well. Aside from this indicator, there aren't any other ways that I know of without pulling the heads and visually examining the seats and valve heads for wear, etc.
If you're still having issues with getting her to run right, maybe it is time to look at the valves anyway.
What does your gauge show for PSI? The best way to measure it is with the carbs removed from the rubber intake boot, and crank the engine 5 or 6 times around. If your not up around 130 PSI then something is leaking somewhere. Sometimes on higher mileage bikes, or bikes which have been run a long time with an incorrect mixture will get heavy carbon deposits on the top of the piston and around the valves, which can prevent the valves from fully seating/closing.
