Oil Smoke Question (Again)

Hi everyone,
I've not had much of a chance to work on or ride the bike for the last few months.
It was starting to smoke before I put it away after the last ride and I'm looking for some advice or tips on pinpointing the problem.
I've checked the simple things:
Oil changd over a couple of times.
New paper oil filter fitted.
New spark plug fitted.
Valve clearances checked.
Timing checked.
Camchain tension checked.
Reed Valve checked.
After doing this, the smoke is still the same.
It appears as soon as the bike is started and doesn't seem to go away after running the bike to try and clear it.
After turing the bike off, some residual smoke still comes out of the exhaust pipe until it clears.
It's definately oil smoke.
Looks like the problem is either the valve stem seals or the piston (oil) rings.
Is there any way of working out which one it is (valve stem seals or piston rings) without removing the cylinder?
I have limited mechanical experience or space to work on the bike and I'd hate to replace the valve stem seals only for the smoke problem to remain.
Any tips / hints / ideas??
Cheers guys.

Cheers, Taff.
I'll try your tip.
Looks like I'm probably gonna have to take the cylinder off to inspect everything.
I suppose It'll give me peace of mind once it's all done.
Oh well, back to the shed!