How do you tell if the nock receiver is left or right?

It has an L or R stamped on the top of the dial.

Sorry I assume you're talking about the indexer?

Yes I am talking about the indexer. Mine doesn't have anything stamped on it.

If nothing is stamped on it I would guess you have the standard nock indexer. I think most users here would concur that having the matching index is not that important. As long as you have the jig adjusted to the proper wing the difference in the index is'nt noticable.

I have 2 Right and one Left  and one with no marks that I use to 4 fletch. I interchange the rights and lefts to do more arrows at once. The difference is negligible, like bbj said above.

It's not a problem when I fletch carbons because I just turn the nock afterwards. But, I am going back to wood arrows and I like the nock turned just to the right of the cock feather. This indexer sets it just to the left. I use a left helical clamp.

The ones with no "R" or "L" marked on them are the straight indexers