I hate to tell you this...I don't think you can reach around there.
Have you looked in the mirror from the back? Bad suggestion, in the case of a beagle you may need to walk over a mirror and look into one of those sleazy mirrors you find on some motel room ceilings. The good news is you will be in full observation of both front and back. Maybe it isn't as bad as you think. Beagle fuzz is nothing like sheep dog shag.Just sayin'
How does it feel right now? Those friendly dog pats from humans might might really slap with no fur.