I consult one of the many animated radar pics when I go out. They also have these for iPhones which I can take with me. After you spend time with these, you get pretty good at timing when the rain will hit vs. how long it will take you to get back to the garage. Each animated frame is time stamped so you can judge the speed of the precipitation.
This is a good app;
http://weather.weatherbug.com/
Enter your zip in the search box. Go to the Radar & Maps link at the top and pic any of the maps. The "interactive map" in animate mode works well for tracking percip. They also have a great iPad and iPhone app.