how to know which apple id is linked to my iPhone

See -> If you've forgotten your Apple ID email address or phone number

If you've forgotten your Apple ID email address or phone number – Apple Support (UK)

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• Your Apple ID is the account you use to sign in to all Apple services. If you've forgotten the email address or phone number that you use with your Apple ID or you're not sure if you have one, you can look it up or check if you're already signed in to an app or service.

• Tap Settings, then tap your name.

Go to settings>your name. You will see your Apple ID there