Well, the two main routes for oil in the cylinder are worn guides or bad seals, and worn oil control rings. Oil rings can be installed incorrectly and pump oil up into the cylinder. A quick evaluation of the overall condition of the rings is the presence or absence of a ridge near the top of the cylinder. If it has had recent work, there should be little or no discernible ridge. If there is no ridge, it does not rule out broken or incorrectly installed oil rings though. if the guides and seals are worn, the stem just beneath the head of the valve will show unusual staining. It should be visible through the intake port of the head. The exhaust valve guide and seal generally do not allow oil in, simply because of the positive pressure in the exhaust side. The intake side however, does have a low pressure relative to atmospheric, and even lower than the crankcase pressure, especially if the PVC valve is not functioning properly, or the rings are worn enough to allow blow-by. Most of the time, a compression check will tell you a bit about the condition of the rings, and with a squirt of oil, can seal the valves well enough to raise the compression pressure long enough to diagnose bad valves.