When I spent time working with a certified mechanic, I was taught that the easiest way to determine if an engine had worn rings was to do a compression check.
While reading a Tom and Ray column in the local newspaper, I learned an even easier method of checking for worn rings.
I now quote from the last portion of the column:
Tom: However, what you CAN tell by removing the oil cap [while the engine is running] is whether the car has excessive blowby. Blowby is created when the rings wear out, and lots of combustion gasses sneak by the rings and into the crackcase, where the oil is stored.
Ray: If there is more combustion gasses in the crankcase than the ventilation system can expel, pressure buildsup in there. When you remove the oil cap, you might see smoke coming out.
Tom: That'd be a sure sign that you'd want to avoid buying that car.
My comment: A compression check is far superior to the easy test mentioned above. It's my guess that if the rings were only slighltly worn--that you would not see any smoke when you removed the oil cap.