Hmm... Well, you have some kind of sensor or film that is sensitive to ultraviolet. I don't know what a good one (or an available one considering that you're probably working with a very limited budget). One thing you could do is use length of exposure to sunlight with and without a glass shield to stop more energetic UV light, but it wouldn't stop all UV, and there are other wavelengths that glass does stop as well which might be important (like infrared).

I ordinarily think of light intensity in units of watts per meter square, but there is also a unit named after Einstein that also measures light intensity, and I'm not sure how much one Einstein is in terms of other Si units. Sunlight at the Earth's surface perpendicular to the sun's rays is 1.4 kW/m², but that covers the entire electromagnetic spectrum, of which most of sun's energy is in the visible, so that doesn't help you. If you assume the sun's ultraviolet output to be constant (which it isn't, but probably doesn't matter for your purposes), then you can just use exposure time, but with the caveat that I mentioned above that it's difficult to filter out all UV and only UV.