By making predictions and seeing that the experimental results match leading to the "most accurately tested theory of physics ever"How was it tested?
Precision tests of QED
The agreement found this way is to within ten parts in a billion (10−8), based on the comparison of the electron anomalous magnetic dipole moment and the Rydberg constant from atom recoil measurements as described below. This makes QED one of the most accurate physical theories constructed thus far.
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