If you upload your data to a website or program on your computer, then it should show the percent grade as determined by your device. Different sites do it slightly different, but RidewithGPS.com shows you the max grade for your ride and where it's at. It also lets you select a portion and see the average and max grade for that too. If you don't have elevation data, then it uses the map data they have. However both map data and your devices data are prone to errors and should not be considered absolute. But overs several rides on the same route you can start to see when you can and can't trust the data.

Strava, Garmin Connect and others do this same stuff...RWGPS is just what I tend to use more often if I upload. For most daily rides I still just use the old Garmin Training Center software to show me the basics that I'm interested in.