A spectroscope is a handheld optical device which allows you to see the light distribution coming from a light source. Should be able to pick one up for under a hundred bucks.
I've actually looked into this in the past (for measuring the spectra of LED bulbs), but they're significantly more expensive than I expected! Gratings are cheap, but digital models seem to run upwards of $2k. Any suggestions for midrange models would be appreciated!
Save money and go analog! Calibrate against a known spectrum. One example would be a sodium vapour street light but there aren't as many around these days.