I'm struggling to find a good picture of it online - but basically the fringe is either woven into the rug, or it's a separate piece that's been sewed to the back.

Higher quality machine woven will look better than poorer quality machine woven (which basically look like an iron on patch) - but in either case I've never seen a machine woven that could duplicate a hand woven fringe, so it's generally the easiest way to spot the difference between hand and machine.