When I used a dual hybrid duos Note 9 it was the first sim that was the first signal bar @Edgie70
They used to have an indicator around the actual signal bar being used but that was removed in Android Pie ! 😞
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.
Sim 1 is on the left. Switch the mobile data on, on sim 1 and off on sim 2 and then swap, and you can see which is which. In the settings you can also label them with your own names which helps when using 2 sims.