Need help if understanding the usage across multiple services

I have my UI service written in next.js. In my UI service, I have a button that redirects to the auth0 endpoint and the user gets logged in. Now the UI service hits some API endpoints. These API endpoints belong to another microservice that is built with rails.

My question is,

  • How can I check if the user is logged in from my API service?
  • How can I get the user details from my API service?