Measure the horizontal distance from Point A to Point B on the highway. Note that the horizontal distance will only be the same as the actual highway distance if the highway is perfectly flat; if the highway rises at all between Points A and B, you must factor out the extra vertical distance traveled.
Measure the difference in altitude from Point A to Point B on the highway. You can use an altimeter to do this, look for road signs identifying altitude, consult a map, or use the altimeter function on a wristwatch or GPS unit.
Divide the difference in altitude from Step 2 by the horizontal distance in Step 1. For example, if the highway rises 50 feet from Point A to Point B, and there is a horizontal distance of 1,000 feet between Points A and B, you have 50 / 1000 = .05. To convert this to a percentage, just multiply by 100; so .05 * 100 = 5.0; there is a 5 percent slope on the highway from Point A to Point B.