I'm having trouble finding comprehensible explanations of how experimenters can ever know that two particles are entangled. I understand that the first experimental confirmation of entanglement used Calcium or Mercury vapor which when excited gave off pairs of entangled photons. But how did the experimenters know that exciting calcium vapour generates entangled photons? I understand that later experiments used crystals that can split a photon into two photons, each of which can be directed by mirrors to travel in opposite directions to distant corners of the lab where experiments can be carried out separately on each member of the pair. But how did the experimenters know that crystal-splitting would yield entanglement among the split photons? Why couldn't they just split yet retain maximal seperability among all their physical properties? What is the simplest-to-understand method for entangled state preparation? Interested to hear your suggestions!