People on here lately have been talking about checking big end bearing wear. I have a quick rough way of doing it by hand. It'll probably only tell you if they're pretty bad but I take the pipe off the bike and the flywheel cover then swing the crankshaft out to about 45 degrees from top dead centre in the direction of the power stroke which is where you'll pretty much have maximum wear on the big end pin then I stick my finger up the exhaust against the piston (you want the piston still completely covering the exhaust port) and push the piston as hard against the back wall of the bore to lock it still then grab the flywheel and try to rock it back and forth. If you feel any play that's not moving the piston the big end or mains probably need replacing. You'll tell if there is play in the mains just by grabbing the flywheel and shaking it up and down.
People on here lately have been talking about checking big end bearing wear