This is why we pass around photo IDs to verify the names listed on keys. But that does not verify the email address on the key.
If you wish to consider the email address on the key as part of the identity that you are verifying, then you can email a secret/password to each email address listed in the key before the keysigning party, and then ask the person to read off the secret/password in person at the keysigning party.
This is particularly useful for people who do not include a name matching their photo ID in their key.
A list of all keys (including email addresses) which will be participating in keysignings can be found on this page.