Law enforcement sources tell TMZ ... the U.S. Secret Service is aware of George's remark, and we're told they're working to see if his words carry any legitimacy as a threat.

We're being told part of the process in making this determination will include an interview with GL in some fashion, preferably a face-to-face. One source tells us George can probably expect a knock on his door soon, and he'll be finding agents on the other side.

The comedian made headlines this weekend after leaving a controversial comment on an Instagram account which regurgitated a story that Iranian officials supposedly recently put out an $80 mil offer for DT's life. GL responded to the news, "We'll do it for half."

He came under fire from both sides of the aisle -- especially conservatives -- with many saying he'd crossed a line with his quip. And, even if he was joking, as he later claimed, the death of an American President is obviously no laughing matter.