The Party keeps watch-- appearances at fund raisers, the right parties on the Hill, who you play golf with, your conversations overheard or direct...you are put on a chart aka ID'd as HOW THEY DETERMINE YOUR LOYALTY..but you are correct in that no one really knows how another votes to be 100% sure...but their probability percentage is pretty spot on.
Contributions is another sure sign, as you stated.
Thing is will this come up in an interview with Senator Clinton and others who seem to think this latest "scandal" *shocked* has never been done before..hypocrites!
No one ever asks Hillary hard questions. If ever a reporter did, he would not be invited back.