I do a blow test on the engine. I pull the engine out of the vehicle, remove the header, keep the plug installed, rotate the crank until the intake port is exposed in the carb. Cover the exhaust port with your hand (wet/oily makes a good seal) and blow in the carb. If the front bearing is bad, you will see and hear it leak. For the back bearing (closest to the rod) I measure the play in the crank. But for this to be of any value you needed to know how much play the bearing had when new. Regardless, when one bearing looks suspect, I replace both.