A flint striker in a butane lighter works as well. My helmet, which has a sensitivity control, can be made to darken with just about any bright light if you make it change suddenly with a head turn, or a wave of the hand, or by turning the light on and off.
Personally, if I didn't have anything easy to test it with, and couldn't make it activate with the lights I had around, I'd just strike an arc and test it with that. Just turn your head enough so you don't have to look at the arc. The light from the arc should still be able to activate the helmet.
All helmets have full time UV protection built in (required by the codes I believe) so you aren't going to do much damage to your eyes even if it doesn't come on - you will just see spots for awhile if you looked at the arc without the auto-darkening activating. The plastic shields built into helmets (and many safety classes) are normally polycarbonate which not only is a strong plastic, but blocks most the UV light. Just wearing clear polycarbonate safety glasses alone goes a long way to reducing UV damage to your eyes, - add them inside any welding helmet and you are more than protected from UV damage even if the auto-darkening doesn't come on.