How do you know if the piston rings in your car are bad?

How do you know if the piston rings in your car are bad?
Worn piston rings will cause your car to burn oil. It will smoke, and you'll have to add oil frequently. Additionally, the performance will fall off as compression is lost past the worn rings. There may be a "slapping" sound as the piston skirts slap the cylinder walls, and this will occur in cases of advanced ring wear.