How do you know if someone is sandbagging?

Played in a local tournament at my municipality today, it’s a casual handicapped event, stroke play singles. This is my third year, was always in the bottom third until this year.

Played one of my best rounds today for a 78, net 69. Incredibly though a guy I was with shot a 79 for a net 64. He didn’t have an incredible swing, but was always in control and didn’t have any really bad shots. A guy in another group shot a 76 for a net 66.

Definitely possible he just got hot, but is there any tells or signs someone might be a sandbagger?