Uh-oh, I have about two minutes to stick in a fact for today.

How's this: Early versions of the oral glucose tolerance test were done with blood from the earlobe.

You can test blood sugar from a wide variety of spots but will get the wrong mix of serum (which has more sugar) and whole blood (which has more protein and therefore less sugar) to compare. However, the most commonly used spots are: fingers (includes thumbs), base of thumb, forearm, toes, heels (in babies that don't walk yet), and yes, earlobes. Toes are not recommended for anybody with neuropathy.
Dr. Bernstein recommends the part of the fingers that are between the knuckles but I couldn't get enough blood there when I tried it myself.