Metal gauge Sizers are used. However it is not possible to check every single bag especially under the time pressure gate staff have. I'm talking from personal experience here as an ex gate agent. If you come across a pax with oversized bag it takes a good 5-10 mins to get the £/€50 payment especially if paying by card. I used to rigorously enforce the 90 bag rule any pax refusing to put bag in hold or pay up was refused travel.