...normally leaking crank seals manifest by white-colored exhaust, but he is right that a simple leak-down test will tell u. See this site for how to do it, or Eric Gorr's excellent $18 book...u will need a ~Craftsman SCREW-in compression gauge and a few rubber expansion-plugs from Lowe's to do the test..."normally" it is the ignition-side seal. Often it can be replaced w/o splitting the cases. Extract it with two long thin sheet metal screws, screwing them into the seal oppostie each other and drawing them out.
Put your new one in your wife's freezer for a couple hours, or spray it with freon, to greatly ease installation. Coaqt it with a light coat of silicon grease will help some too.
Used to have an RM350 (to race the open class with) that literally ate those things...like most RM's it was a joy to ride as long as u didnt mind it aging much quicker than other bikes do...