We sailed on the Magic on the last voyage prior to cruises being cancelled. They were doing the temperature checks as we were boarding. I knew I was fine but it was still nerve wracking. Someone in line suggested my son take his cap off because that can make your head warmer. Just little things like that. Seems like they did have people step aside and wait if there was concern so they could retest and I am sure they have the protocols as PP noted. But it is certainly a valid question. I was worried and I knew I didn't have anything to worry about.
From what I've heard from some WDW bloggers is that if you fail the test the first time you go to a tent to have it done a 2nd time after some time has elapsed. I assume that would be the same type of process for most.