20/20. They're fairly easy to differentiate. The curved ends (finials) of e, c, C, r, S, s are generally horizontal in Helvetica and at an angle in Arial. The tail of the uppercase R and ascender of the lowercase t are the biggest giveaways. Funny how they are so similar, but Arial is pretty tacky in comparison...
When looking at uppercase I could see such a difference in kerning, Arial just can't get it right.