Although it may take some physical distortion of your head and neck, you can always pull the exhaust and look inside the cylinder at any time to check the condition of the piston, rings, and cylinder. This is really just a short cut to pulling the head and cylider though. If your bike becomes harder to start but kicks over easier you are likely ready for a new top end.
FWIW, pulling the head to check the cylinder only costs you time and possibly a gasket. If you pull the cylinder, you'll need to replace rings as well. Too me, if you go that far, you should just do the entire top end. Just my $.02.