You can visually check them by looking at the boots on the inner and outer joints. If they are wet or slinging grease everywhere then that's the one to change. The only way I found I had a bad pass. side inner joint was by accident. I leaned on the shaft while doing something else under there and heard a clunk. Pressing down on the shaft showed I had play in the inner joint that was causing a horrible vibration on accel and any speed above 50.