You really can't.
Unless you email a code to the user, and they have to check their email to confirm.
You've probably experienced that tons of times.
You can validate it though.
Make sure the structure is correct email@example.com
with the @ and . in the proper order and they exist.
Another way is to extract the part after @ and ping or try to open
a file on that website. Usually, the part after @ is a domain.
Like: @yahoo.com or @msn.com or @gmail.com
You see if the "email" domain part is valid.
If I were to enter "firstname.lastname@example.org" ... dfslkjf.com is not a valid domain,
so I'm probably lying about my email address.
That's about the best you can do.