You can use simple heuristics to get a very good idea.

1: How high in the sky is the sun? (Many other factors like time of year, time of day, and latitude feed into this) 2: What is the chance for you at a given time of the sun being blocked by clouds, buildings, etc?

UV index weighs UVB more heavily than UVA. UVB falls much faster than visible sunlight as the sun gets lower because photons travel through increasingly more ozone and atmosphere. So the higher in the sky the sun is, minus cloud clover, the higher the UV index is.

I don't think so. If you controlled for temperature and season I bet you wouldn't be able to tell.