What is the most common way for human beings to authenticate a person’s ""identity"" in daily human life?

Something a person "is," like the way he/she looks or speaks.

Explanation:

When we see a person for the first time we attribute different traits to them and this determines our behavior towards them. Human physical appearance determines their first impression and social relations. And the next comes how one speaks. These factors formed the basis for the interaction with the person.

if you are on the computer you can not take a picture but you can upload one but i am not sure about phones but i'm pretty sure it's the but to upload one just click the paper clip looking thing/ link thingy thing and upload a pic