A lot of the advice here tells someone how fast they covered a certain distance but I think the OP asked how to tell how fast you're currently running.

If you cover a mile in 8 minutes, it doesn't mean that you ran at an 8 minute pace the whole time. Many experienced runners can tell you that they went out and did 8 min/mi for the first 15 min and dialed it up to 7 min/mi for the rest of the time. Back in HS, we used to plan our runs before we left...like, "We'll do the Signal Hill run at 6:20's (I'm a lot slower now).

Of course, this takes a lot of running experience to be able to do this. That's why I recommended the Garmin Forerunner. If you want to see your current pace, you look at the display.