How do I know if someone’s using a dual SIM card
How do I know if there is a dual Sim card on iPhone
Question marked as
Are you asking about your phone? Or someone else's?
If you want to know about using dual SIMs, see here:
Using Dual SIM with an eSIM - Apple Support
Using Dual SIM with two nano-SIM cards - Apple Support
If you want to know if someone else is using dual SIMs, ask them.