Symptoms of engine wear out...
Eating oil, smoke from silencer, strange sounds from engine etc.
If ur bike is eating oil.. it would give smoke, it might not be visible at idle RPM, but give heavy throttle to a standing bike and u can see grey smoke in minor cases.. at extreme black smoke.
at Idle if ur bike makes gud gud gud gud sound... jese dhakay say chal rai hai.. it might mean crankshaft bearing is loose... which means trouble in near or future.
grinding sounds mean trouble...
clutch plates wear out when ur bik's rpm and sound is normal but its speed is not upto the mark.. specially pick up is bad.
U can even hear grinding sound when u move the bike in 1st gear with heavy throttle... grrrr... power loss due to clutch slipping...
when clutch plates are gone bike wont alos get to the full speed and would get slow while climbing overhead bridges etc wont maintain speed..