Most commerical detectors (e.g. Radex) only detect gamma. You need specialised devices to measure alpha, and usually the devices that measure alpha only measure alpha, not gamma and alpha. Reason being that we generally "don't care" about alpha radiation, given it only penetrates a few millimetres away from the source. The large alpha particles are block by air, the watch case, and skin. It is believed to largely be biologically harmless (as long as you don't ingest an alpha emitter).
Generally if you have a radium watch, you only care about the gamma emissions. That's why tritium started being used in the 70s. Tritium emits 100% alpha, so it's considered much safer (and its half-life is 12 years, so this also contributes to the "safety").
Click to expand...