How to identify who is sharing my Apple device?

Go Settings > [name on top], and scroll to the bottom. The devices you see there are signed into your account and that data is being shared. Simply tap on them to delete and change your Password so they cannot sign in again.

Without any further information, there is not much direction that we can give you other than just guessing what you are talking about.

Why do you think your device has been "shared"? Because of all the security features Apple has around the system on the device this is incredibly difficult to achieve. More likely is your account may be compromised, but even that has been surrounded by a number of security features.

In addition to the previously posted link about what to do if you Apple Apple Account has been compromised:

Read this document if you think your Apple Account has been compromised. --> If you think your Apple Account has been compromised - Apple Support

For iOS 15 or earlier, if you want to see if anyone else has access to your device or accounts, click here --> Checklist 1: Limit device and account access - Apple Support

For iOS 16 or later see how Safety Check on iPhone works to keep you safe. Safety Check for an iPhone with iOS 16 or later - Apple Support

Use the information in this document to check your Apple ID device list to find where you're signed in --> Check your Apple Account device list to find where you’re signed in - Apple Support

Related materials:

Personal Safety User Guide

Personal Safety User Guide - Apple Support

> open the Table of Contents and review the articles

A document with general information about security and your Apple ID --> Security and your Apple Account - Apple Support

What makes you think someone is sharing your device?

Sharing in what way?

What exactly is happening?

If you explain what issues you are having someone may be able to advise.