It is HIGHLY unlikely that any outsider could "infiltrate" a standing force like the NG, much less the more highly active and trained military outfits. Personnel in units know each other, know the "language" and customs, and would readily recognize any deviance. A little thing like a non-regulation haircut or a small detail like improper equipment or clothing could give it away. And comms protocols, and countless other things that an infiltrator wouldn't know about.