Easiest check is to run a piece of paper around the tail light. If it is able to go all the way around with being stopped/caught up, it's probably okay. Mine had a few spots where it was right up against the body, and therefore no way to let the paper through.
Vehicles: 2012 Focus SEL - Ingot Silver - +MyFord Touch
Mods: Drop in K&N Air filter, 30% Tint
Build Date: 12/19/2011