The icon screen sample is pretty low res. It's much easier to see on an actual retina display, but the differences are indeed subtle.

A couple of differences: The 'a' is clearly different, and you should be able to spot the difference even with an untrained eye; the 'C' and 'G' are clearly slimmer in SF.

The curves of the rounded letters have a subtly different feel to them matching the names of the fonts: Helvetica is sterner and more geometric, San Francisco more inviting and informal. You may find this hard to see is you look too closely, but see if you can feel it or learn it by attaching adjectives to the look of the words.