Ruining the game for everyone; two things I love about Helvetica
— Terminals at right angles to the stroke. http://c.jon.gd/image/3Q0y2u323j3C . Arial looks particularly sloppy with jaunty terminals. It is possible to have a similar grotesque sans-serif feel with offset terminals (see Univers & Akzidenz Grotesk) but they're a crucial part of what give Helvetica its character.
- The uppercase R. Has a really strong leg compared to Arial's half-assed flaky leg.
A thing that helps me distinguish them is that Helvetica is a bit more decorative with some of the capitals, like G and R. I think the Arial designers were probably trying to make it "even more Helvetica than Helvetica", by simplifying the flourishey/decorative bits, like the bottom thing on a G and the curvy bit at the top of the slanty line of the R. (These are the proper typographical terms, btw.)
Kerning. If there's more uniform spacing between all letters in one picture it should be the original: "OTA" in Toyota, "THE" in The North Face and "TEL" in Mattel. But it would be hard without seeing them both at the same time.