The least expensive method of determining the gradient of virtually anything is with the use of water.
Water seeks a level automatically.
Then each clear hose can be attached to a common garden hose. The length of the garden hose does not matter, nor does the terrain in between the two ends of the hose, as long as it is not higher in the middle then the ends of the hose. Make sure all connections are tight and do not leak. Then fill the hose full of water, letting it settle and air bubbles escape, until the water maintains a level visible in the clear plastic tubing. It may be necessary to raise the lower end of the hose if water pours out, until it too maintains a level that is both visible and stable.