To measure the vertical fall of your water source, you will need some form of a level. The preferred instrument is an optical sight that has a level built into it. This device allows one to sight a level line while looking through the optical sight. One such level is made by CST/Berger and is called a "Locke Hand Level" (around $20). If an optical level is unavailable, a short carpenter's level may be used. Let me show you how...
First, measure the height of your eye level (from the ground to your eye). Then start at the bottom end of your water source. Sight a level line and find where that line intersects the ground upstream. Have a second person mark that spot. Then stand at the marked spot and sight another line until it intersects the ground further upstream.
Repeat this, counting the number of times you repeat it, until you reach the highest point of your water source. Now multiply the number of times it took to reach the source by the height of your eye-level. The result is your rise or head (Example: eye-level is 5 feet 6 inches. It takes 6 sightings to reach the source. 5 feet 6 inches times 6 equals 33 feet of rise or head.)
It's just that simple! From here you can measure the flow of your water by reading this blog post: Water System Basics.