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.