> Either shake your head from side to side or wave your open hand in front of the lights to test if there's a strobe effect.

My method is filming any suspected bulb (not just Xmas lights) in slo-mo for a second and watching if they flash on playback. However, most I can tell out of hand without resorting to slo-mo.

If there’s something untrustworthy about the light emitted, it’s an LED.