I get this every year, since during the school year (I'm a teacher) I have a hard time finding time to ride regularly, but during summer I ride a lot! The way I gauge it is by noticing which gears I select for sections of my normal rides. There are only two roads out of the little town where I live, so if I ride from home I only have two choices - north or south. I tend to ride the same routes over and over on training rides. I've done the portions enough to be very familiar with which gears I use for certain sections. When I'm in shape I seldom use my granny. When I'm on my bike for the first time after a few months and feeling very weak I resort to the granny quite often. When I'm out of shape I get tired and want to rest. When I'm in shape I do the same ride without stopping, feel great, and feel like "that was easy."