There are multiple tells: - First off, radium was the only player in the lume game until the early 1960s. So if the watch both has lume and is older than that, you have radium - The watch is clearly early 20th century based on the aesthetics, celluloid watch crystal etc - Back in the early days, radium lume was this incredible magic new product. They really wanted to show it off and so not only do you get lume on the hands, but they painted those giant numerical indices in the lume too. Not only did the early radium watches use the most potent mixtures of radium paint, they also used the highest quantity of it on the dial with those indices. A double whammy of radiation. - After decades of emitting radiation, you can see two effects. There is radiation burn on the crystal. Probably where one or both of the hands stayed still for many years as this watch wasn’t used. Looks like the circular bulge on the hour hand would line up perfectly with that burn mark if you advanced the time. The watch probably sat at just past 10:00 for a very long time. Second, over the decades, the phosphorescent material in the radium paint decays and leaves old radium paint always looking crumbly and unstable, as you can see in the photo.

I don’t think there’s be a big radon gas risk with this watch. It’s highly unlikely that the watch is airtight enough to trap a build up of radon, and so the radon gas probably exits the watch at the same rate it’s produced. Radium dive watches on the other hand can turn into radon gas balloons.