I've made a cool discovery: cellphone cameras (and probably most cameras with a live display (sometimes called "live view") can be used to observe time-varying light sources.
Simply open the camera app and point the camera at the light source of interest, then rock the camera rapidly up and down so that the light source paints a trail on the screen. In most cases, you'll be able to see the trail breaking up into dashes or repeated images, showing that the light source is time-varying. It's not as good as eyeball- or mirror-jiggling (see my "How To See" page), but as a quick way to see if a light has time-varying behavior, it can be useful.
I've tested it so far on three cellphones: LG G4, Apple iPhone 6+ and Motorola Droid Mini. It helps to let the camera focus on the light source first before rocking the camera up and down.