The ribbon snake is below and facing right; the garter snake is above and facing left. You can see some of the differences: the narrower head and body, for example, and the cleaner pattern (ribbon snakes usually don’t have markings between their stripes).
Ribbon snakes have a pure white jaw and a stark white dot in front of their eye.
In your pic, it looks yellow:
Also, there are markings between the longitudinal stripes, which are indicative of a garter.
But when we take a closer look at their heads, you can see even more of the characteristics that tell them apart.
As you can see, the garter snake does not have a white spot in front of its eye. And take a look at its labial scales — i.e., the scales along its upper lip: dark markings between the scales that make each scale stand out.
Yours looks like this one to me.
The ribbon snake, on the other hand, clearly has a white spot in front of its eye. (Racers also have this, by the way.) And there are no dark markings on its labial scales at all.
This does not look like yours to me.
I think that it’s their faces, more than anything else, that make it easier to tell these two species apart at a glance. After all, garters can be skinny and ribbons can get fat (or at least pregnant), and snakes can always lose parts of their tails, but garters won’t grow white spots in front of their eyes and ribbons won’t grow dark marks along their lips.
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