Easy way to tell if its the plastic or glue...
On the NGC coins that I have purchased and submitted, I have had several with 'nibs'. At most I have had 6 locations where the sealer has left the nib, one at the top and bottom, and two on each side. I find that an x-acto knife works good to remove the excess plastic. One thing I take a look at, esp. with purchased coins, is the interstitial space between the two halves. If the coin is NGC sealed, the plastic looks the same throughout (barring voids). If it is glue, you will notice the material may be softer/harder, different color, or other irregularity.
Fortunately, I've never had a reholder rip-off, but have built many model airplanes to know the difference.