Pull a vent. Take your smartphone, turn on the camera to video mode, turn the flash to "On" (not auto). Stick the camera end of the phone up in the vent and do a 360. Pull out phone, stop video and review. Should tell you if you have any critters up there.
With that said, like another poster indicated, we still get foam pellets here and there on our 3 year old Big Country. Ours tend to congregate in the light fixtures.