There's no easy way Doug. You obviously know the original owner and can be pretty sure of the mileage. You could also probably ask about previous works to the engine, if any. I've just got hold of a 1980 T140E that's been laid up in a greenhouse since 1986. The original owner was rebuilding it and it looks like he got ill as he was removing the crankshaft. One rod was removed and the other was still on the crank. As the crankshaft was already out it was easy to get the plug out to have a look at the sludge trap. Here it is You can see that it's pretty full. If this blocks up then no oil gets to the big ends and something horrible happens. I'm not trying to be the doom and gloom merchant as I've also taken a '76 engine apart recently which was rough but the sludge trap was really clean. I've also had engines that had been supposedly 'rebuilt' and no one had been near the sludge trap, another where the sludge trap plug was only finger tight but the trap was clean. It's a lottery. If you are pretty sure it's been looked after and the oil regularly changed it'll probably be ok for many thousands of miles. I've known of some who install an oil filter and use a thinner modern oil to try to clean out the sludge trap while running. Modern oils are designed to hold impurities until they get removed by the filter. They run the bike for a hundred or so miles and change the oil, and repeat while inspecting the filters closely to see what it's picking up. But - all those impurities have to get past the shells and journals of the big ends to get out, if they get stuck.... You can run the bike and filter what's coming back to the tank onto some linen and see what's moving about in the oil. At the end of the day it's really up to you. I'd bet it'll be ok to run as is and just keep your eye on the oil light. Me? well, I just like climbing into these engines for the fun of it, ask @darkman and @speedrattle. Shame I don't live a bit nearer to you............over....
Iron, thanks again for all of the details, I wish you were local too! Since the motor has such low miles (15k) and that the previous owner was diligent on oil changes, looks like every 1,000 miles according to their hand written log in the user manual, I am going to chance it and leave not do a rebuild but I'll certainly change out the fluids. Thanks!
Doug Shebesta. What i would do if it were my bike and usually do on older bike is run a good oil say for 100 to 150 miles flush it out and refill with new. Whilst doing that check the wire mesh filters one in the frame and one just under the right side of the engine that will tell you if you have any debris floating around in there. Perhaps with both initial fill and again with the refill. But that's just me. Joe.
Doug Shebasta. That is just my way. I will strip an engine when it needs stripping and you will know when or something goes wrong. Joe.