Store front api call to Custom app customer verification

[LittleIt]

Hi guys, 

I am building a custom app for shopify, in the store front i want to do a call to my custom app to store some data. But i want to verify which logged in customer did the api call. How can i know which customer did the api call? Is there a access token that i can pass from the front store with the api call and in my custom app i can verify that it is indeed that customer that made the Api call?

Can someone help me? And explain me how i can check in my custom app which customer did the api call to my custom app?