How can I find out if it's going to frost tonight in Denver?

Weather.com. Type in your zip code.

@gailcalled Have you found them to be accurate?

For my area, I have been pretty pleased. You can get an hour by hour forecast. It’s never a perfect science however. The site will give you a percentage for – say – chance of rain or snow

I find that looking at my outside thermometers, one in the sun and one in the shade helps. After dark, if temp.drops to low thirties, there is always a chance of frost at sunrise.