How to determine the speed of my 1989 ford f150 automatic with a 302 v8 motor in it?

I bought the truck from my grandpa a couple of months ago & I literally know nothing about the truck other than it's a 2wd.

Old way was timing yourself on the highway from one mile marker to the next. Hold the vehicle at exactly 60 mph you should go a measured mile in 60 seconds. GPS has made it so much easier now to know how fast you are going and if your speedometer is right.

You can look at the speedometer when you are driving it. Or you can get an app for smart phones that use GPS to determine the speed of the vehicle.