Answer:
To the limit of our ability to measure with accuracy, no right triangle has ever been measured that doesn't fit the theorem. When one is found, we'll know that the theorem is false.
This is exactly the method of science: A theory matches what we see and makes predictions that check out. When something comes along that doesn't match, we either modify the theory or completely throw it out. A theory can never be proven, but is always vulnerable to be disproven, no matter how long it has been used. All you have to do is find the example where it doesn't work, and you'll be famous.