This is a follow-up to my earlier question. That was about telling Voldemort about the Foe-Glass device, this one is about using the Foe Glass while at Hogwarts.
Barty Crouch Jr., while impersonating Alastor "Mad-Eye" Moody, had Moody's Foe-Glass in his office during Harry Potter's fourth year at Hogwarts. Near the end of the book, Goblet of Fire, the Foe Glass shows Albus Dumbledore, Minerva McGonagall, and Severus Snape just before they burst into the room to arrest Barty Crouch Jr.
This proves that all three were enemies of Crouch and therefore of the Death Eaters too. So Barty Jr. had an entire year to look through the Foe-Glass and know Snape was not really a Death Eater. It would be a major oversight for him to not check out friend and foe around Hogwarts.
Could he have used the Foe-Glass to learn Snape was not a true Death Eater?
If he did, he would have warned Voldemort and the other Death Eaters.
Edit: Not sure why this was flagged as a duplicate of my earlier question. As mentioned above, that question is "Did he tell Voldemort about the Foe-Glass?" which is different from this question "Did he use the Foe-Glass to learn Snape's allegiance?" It's quite possible for Barty Crouch Jr. to tell Voldemort about the Foe Glass and not learn about Snape's allegiance. It's also possible for him to learn about Snape's allegiance and not mention that or the use of the Foe-Glass to Voldemort. Neither condition implies the other. He could do both tell Voldy about the Foe-Glass and learn Snape's allegiance, or he could do neither.