Here is how to find it:
1. Stand up straight and let your arms rest comfortably by your sides.
2. Make a fist and measure the distance from the floor to the bottom of your closed hand.
3. This number should be the height of the face of your anvil.
The reasoning behind this is related to the geometry of the anvil and hammer surfaces as you bring them together by hammering. An anvil at the proper height will receive hammer blows that fall flat to its surface. If the Anvil is too low, the hammer will fall at an angle. If the face is too high, it will produce an angle in the opposite direction. The trouble here is that angled blows create dents in your work, which can be annoying and difficult to clean up. Working at an anvil of improper height will also force you to compensate by changing your body posture. This can be uncomfortable and potentially damaging to your body and your work.
So, if you are getting started in the world of metal work, then take the time to make the right adjustments while setting up your anvil. It will save you a lot of trouble in the future and reduce your learning curve.