Trucker's (trucker's) tan is far more commonly used than "
trucker's arm". It's used when recognizing that a person's left arm (driver's side arm) is tanner/redder than his
right arm.
It's used in the same way a person would be said to have a "farmer's tan" if both his forearms were tan/red up
to the point where his (short) shirt-sleeves end on the upper part of the arms.
With the ubiquity of
air conditioning in cars today, "trucker's tans" are seen much less
frequently. But if noticing a person's left arm is red/tan (but not the right), say, "Hey Joe, you been driving a truck? You got a 'trucker's tan', my friend!"
In the same way, upon noticing
the whiteness of a friend's upper arms in contrast to their forearms, you might say, "Hey Joe! You been out farming today? You got a 'farmer's tan', my friend!"