It has to start with check-in staff...oh, wait, we don't have them any more with QF and J*, (Virgin?)do we. Just staff assisting people to use the automated check-in. Or people do an online check-in beforehand. So they arrive at the gate unscrutinised, and the gate staff don't pull them up. Not that check-in counter staff are blameless--I was sitting near the gate waiting for Skywest boarding in Perth last year, near a woman carrying three large tote bags. The gate attendant spotted her, walked over, and asked if she had checked in downstairs? Yes, was the reply. The incredulous attendant then directed the offender back downstairs to check in some of her excess. I felt like cheering.
