If the hairs are folded back and held away from the edge lace, at this points scissors will work just as well to trim that edge.
Do it with the top of the piece facing you and you'll see which bit is lace with no hairs (or just the short stubs) and which bit shouldn't be trimmed because it still has the full length hairs attached.

But for anyone else thinking about cutting their piece down to size:
Scissors cut from the top and the bottom, so using them will always cut in to the hair when cutting the lace.
By using a blade on the underside, you're only cutting the lace and leaving the hair still attached to it.
Always use a very sharp craft scalpel.