Some wear marks on the external cover fasteners may give a clue something has been done it, but to know for sure you would need to get in there yourself.

 

Gotta take off the outer cover and flywheel. No other way to see.

 

If you like to wrench, pulling the cover off to see is simple.
The gaskets aren't horribly priced.
A 1/4 drive in lbs torque wrench is your friend on klr

 

How well does the side cover gasket fit.
Usually if someone has been in there the gasket won’t look OE and it’ll be green.
And fastener wear,does it look like somebody chewed on one or more of the cover bolts.

 

Too many owners use too much RTV silicone sealant on their used/torn gaskets rather than scraping off the remains of the old gaskets and purchasing new gaskets.