Arial versus Helvetica. How to tell them apart. Is Arial just a poor copy? | I love typography, the typography and fonts blog

I find the easiest way to to tell arial apart from heletica is that in
any lowercase letters that end in curved sections, ie acefgjrsty are
different: arial ends them at angles, and helvetica ends them straight.

http://ilovetypography.com/2007/10/06/arial-versus-helvetica/