Thermal cameras aren't accurate enough for a medical diagnosis, but good enough for an initial screening.
Those flagged by thermal cameras would then need an medical grade IR camera that is properly used. These cameras require proper distance, dry skin, and takes into account the ambient temperature. This would almost certainly be done indoors or under a tent. Wipe your brow first.
This would then flag people with real fevers who then become candidates for what will be hopefully just a 15 minute swab test.
Before a certain poster reiterates their concerns voiced in another thread: when you're out in public, you can be photographed and videographed and your conversations recorded. When you act it public, those public acts have no 'rights to privacy.' The same would be for the part of the light spectrum in the infrared range. If you don't want people to see what you look light in infrared, stop radiating heat when you're out in public!!
Disney already scans us for electromagnetic radiation with metal detectors and wands. They already go through our bags (which police can't unless they have cause or a warrant). They scan our finger print (yes, I know, for isometric readings only).
Disney can deny entry to anyone they want for whatever reason they want (except for a few protected classes, which doesn't give those protected classes carte blanche to ignore other reasonable barriers).
Or just let people in willy nilly even if they're coughing up a storm.
But to what end is all of this? There's a great number of asymptomatic carriers (either those who never get sick or who have not done so yet). These are the people we need to be worrying about.
If you read first-hand accounts of people who have had the "mild-to-moderate" version of this with the high fever, they can barely get out of bed, let alone a theme park.
The reality is that whatever they do will be 90% for show.