Every time you leave the US (and when you come in), the airline submits a passenger manifest.How does the CBP/CIS know how much time an LPR is absence from the USA at a port-of-entry, a reply from the PR?
Every time you leave the US (and when you come in), the airline submits a passenger manifest.
They have your passport details, personal details and in case of some airlines GC details.
That is sufficient to identify your departure. When you come back, your GC gets swiped and that records your arrival (plus the stamp they place)
and like JackO said, stamps in the PP from other countries would help with this as well.