Primarily by email address validation.
For any document signed via Invite2Sign or signed by yourself from your account, the Signer has proven ownership of their email address. Most of the time this is sufficient, and probably more secure than posting a document by snail-mail - how do you know it was received by the intended recipient, without using FedEx, or similar, who mostly use electronic signatures for verification anyway? And it's more secure, partly for internal security reasons, to emailing a document to be printed and signed (who actually handles the printing and scanning/posting?).
Sometimes you may want more proof of identity. In this case you can opt to use Mobile Phone verification as well. We send an SMS text message containing a PIN Code, to the Signer and they must enter this code before signing the document. This means the Signer has proven ownership of both their email address and mobile phone number. This technique is often used by banks and other financial institutions to verify your identity.
Please note that with In Person signing we have not verified the Signer's identity, unless you opt to use Mobile Phone verification, and so you should take steps to confirm their identity.