One way I was told to tell Beck from Costanza are the ears. Beck, as you can see in this first sample, puts sort of a little "w" inside his ears, and the top line is usually short, looking like a little "c" on its side:
http://img37.imageshack.us/img37/1935/captainmarvelbeck.jpg
Costanza on the other hand makes the top line inside the ears long, following the contour of the outer line.
http://img515.imageshack.us/img515/4947/captainmarvelcostanza.jpg
I'm sure that there are probably other/better ways, but that's how I was taught (I think by either Jerry Bails or P.C. Hamerlinck).
-Eric