Apart from the extra wing fence and angled leading edge of the MiG-17, what are the differences visually between the two aircraft?
In addition to the elements on the wing mentioned by @Jamiec, on the rear, there are some little differences close to the rear air brakes.
See the picture below of a Mig-15:
The rear air brakes are in dark grey on this plane, the fuselage is smooth.
On a Mig-17, there is a small protruding part on the closed brakes; see this picture:
I cannot find if this change on the mechanics of rear air brakes is on all versions of Mig-17.
I'm surprised no-one's mentioned the length - the MiG-15 has a much stubbier appearance than the MiG-17, because it was rather shorter (10.10m vs 11.26m)
The wings of the MiG-17 are angled at 45°, rather more than the MiG-15 (helpful to identify from above), and the tailplane is has a small amount of extra rake too.
Overall, the MiG-17 has a longer, sleeker profile than its older cousin. Here's a comparison, 15 in blue, 17 in red.
Visually, the only differences are those you've mentioned - the extra wing fence and the angled vs straight leading edge.
The MiG-17, being developed much later was meant to be a far more advanced aircraft compared to the MiG-15. But visually, they were very similar indeed.
In addition to the greater angle of wing sweep and the additional wing fences, the wing of the MiG 17 had a different, thinner profile with a thickness/chord ratio of 1 to 10 or 10%, down from the MiG 15's 11%. This was still relatively much thicker than the 5% wing used on the supersonic McDonnell F4 Phantom. The superior efficiency of the MiG 17's wing at subsonic speeds, coupled with the lower wing loading made it more maneuverable than the F4 in the horizontal plane. The MiG 17 had increased power from a developed version of the Nene engine giving 7,450 lb thrust with afterburner. Early MiG 17s had the MiG 15's armament of one N37 37mm cannon with 40 rounds and two 23mm NR23 with 80 rounds each, but many later aircraft substituted a third NR23 for the 37mm gun, giving a combined rate of fire of around 2,500 RPM with ammunition increased to 100 RPG.