Uhhhh the big toe section?
They're also labelled L and R
That's pretty much how I could tell (big toe), but when I stuck my boots in the opposite ones they still fit.
Where would the L and R be on the binding? I looked all over both of them and couldn't find it, as I figured that would be the easy way.
As I mentioned, I'm about 90% sure I got it correct, but thinking about trying it out tonight and thought I'd double check