Originally Posted by Sewnoma

I've never seen a machine where the thread would go into the needle from the back to the front, so if the needle is set with the eye facing forward, I think you're safe threading the needle from front to back.

For needles set with the eye facing to the side, the thread will most often go into the needle in the direction opposite of where the last thread guide put it. So if in threading through the final thread guide you swing the thread from right to left, you're probably going to thread the needle from left to right.

Those are the two "rules of thumb" I always use for threading an unfamiliar machine. As others have said, you'll know pretty quickly if it was wrong, and then you can either just try it the other way, or look it up.