You can't.
You might be able to do a DNS lookup on the domain given but that's only part of the story and a very small part.
If you were to enter
BillJoy@Google.com as an email address a DNS lookup would tell you google.com exists.
Nothing except actually sending an email to the address and seeing what response you get (if any) might tell you whether the address actually exists and is monitored.
But if you get no response it doesn't mean the address exists. Many mailservers will not send a response when they get mail for a nonexistent address because so much of it is coming in all the time (spam, virus emails, etc.) that those responses are flooding the network.
Those that do send a bounce may send anything at all, so you can't rely on a standard format.