"E.g. if Russia just put one on a satellite and didn't tell anyone, does anyone have any way to notice?" My wild guess would be, a bomb radiates and a satellit

"E.g. if Russia just put one on a satellite and didn't tell anyone, does anyone have any way to notice?" My wild guess would be, a bomb radiates and a satellite usually does not, so it might be possible to easily check new launched ones or all of the satelites of a certain size?