How do I check the differential fluid on my 2002 Chevy Silverado?

You have to get under the vehicle. On the rear differential the plug is in the front of the axles on the passenger side. On the front differential it is on the drivers side and should be a very large plug about 1" or 22mm head size. This applies for both - pull the plug and slide your pinky finger in it should be no more than 0.25" (1/4") below the bottom of the threads on the plug.