These are two quick, easy, and incredibly inexpensive methods to measure nip footprint length and CD variations. Like most nip footprint measurement, these are static, single condition measurements (i.e., the nip opens and closes, but does NOT rotate).
Using tape, lightly place strips of tape down on the open nip roller, intentionally leaving bubbles in the tape-roller contact area. This works best with tape on a chrome roller. After applying the tape, close the nip to the desired setting, then open it and measure the MD distance that the bubbles are ironed out in the contact zone.
Using sticky notes (such as 3M Post-It® notes), first, close the nip at the desired settings, then insert the sticky notes in from either side of the nip footprint as far as they will go, one note from either side (upstream and downstream) and tack them down to the roller. Next, open the nip and measure the MD length of the gap between opposing the sticky note pairs.