Time for new rings?

Is there an easy way to know when it's time for new rings on a 2-stroke? When is it too late to replace the rings? Does the engine sound change or is there somethng else to look for?

I have 2 pw50's that the previous owner let go way too long. They both have some piston slap going on. I am having one bored out and a new piton kit put in. The other I'm thinking of just replacing the rings but it may be already too worn. The manual says to replace the rings when they are out of tolerance. My question is how can I tell without tearing the engine apart.

Thanks for any advice.

I did a compression test and it was about 90 to 95. It runs pretty good with our 6 year old on it but I can hear the piston slapping. It will usually start on the first kick. I just don't want to wait so long that it needs major work. Can I just replace the rings or does it need a complete top end?