Use a CD disk - really - it disperses light similarly to a dispersive prism. You can then see and estimate the amount of red, blue and green in a light. It works very well if you just want to reduce blue light sources at night.

Take a photo

Implies no white balance shenanigans

A sheet of paper will generally manage ok for white balance calibration. A 3 tone grey card would be better, but a single, simple white will suffice