I am aware of the Bell experiment that is supposed to demonstrate that entanglement exists.
The only differences between photons, that I am aware of, are frequency and energy. So, in the Bell experiment, if you use the same energy and frequency when the two photons are emitted you would expect them to have the same characteristics when they are detected at a distance. If one of these properties is varied between the photons when they are emitted then there would be differences at the detectors.
But how does any of this determine that these photons are connected (entangled)?