A driver's tan is associated with extensive driving while hanging one arm out the window. The biker's tan is a tan line that goes up 3/4 of leg, where lycra bike shorts would normally begin to cover.
Some of the common tan lines associated with a farmer's tan include:
forearms (from a short-sleeve shirt or t-shirt)
neck (from shirt collar)
thighs (if shorts are worn)
ankles (from socks, only if exposed)
forehead (if a hat is worn)
Also, depending on the activity, the inner side of the arms may be paler than the outer side.