How do I know which line is called, primary or secondary?

You can use the following information from Using Dual SIM with an eSIM - Apple Support

to check (and to change) the labels:

After your second plan is activated, label your plans. For example, you can label one plan Business and the other plan Personal.

You'll use these labels when you choose which phone number to use for making or receiving calls and messages, to designate a number for cellular data, and to assign a number to your contacts so you know which number you will use.

If you change your mind later, you can change your labels by going to Settings, tapping either Cellular or Mobile Data, and then tapping the number whose label you want to change. Then tap Cellular Plan Label and select a new label or enter a custom label.