I’d also like to know how colleges actually determine the “low income” status of a student, for sure, at schools with a wall between FA office and admissions. There are some markers that are pretty clear - fee waiver for apps/tests for instance.
Making the GC aware (if not already aware because of reduced lunch or fee waivers processed by GC) certainly can help so s/he includes that info in rec letter. Of course not all schools have GCs able to write comprehensive recs that would include that info, that may be another benefit for lower income kids in wealthier schools, whether by scholarship or magnet status or geography.