If you can't get the information from the previous owner the only way is to have it checked. Checking and adjusting valves is often neglected because it is relatively difficult and expensive. Speaking from personal experience I checked the valves on my '95 ZX-6R at 15,000 miles and 7 of 8 exhaust valves needed adjusting and a couple of intakes. I could tell that the valves had never been checked.