The free option: use your smartphone's flashlight and try to spot unnaturally bright / colored reflection. If you want to spend some money and don't mind carry

The free option: use your smartphone's flashlight and try to spot unnaturally bright / colored reflection. If you want to spend some money and don't mind carrying an extra device: probably any of the "hidden camera detectors" on the market will be at least somewhat useful. K18, CC308+, etc. We're trying with our work to get better or at least equivalent results without having to use external devices.