To check your crank case seal on the clutch side of the motor pull the crank case breather tube out where you can put some spit on the end of the tube with your finger while the engine is running, If It blows a bubble the main seal Is likely bad. To check the main seal on the mag side pull the mag cover and mag and look at It If it is leaking It will be visually obvious. This is the poor boy method, before you actually take the engine apart you should perform a leak down test. It Is also possible to have a leak between the cases and suck oil from the tranny that way. kx500's are also known to develop leaks in the cast erea around the main bearings. while performing the leak down test you can spray soapy water on the erea oround the main bearing to look for bubbles.