How do you know if someone is tracking your Android phone?

Open your phone’s Settings app.
Select Network & internet.
Under SIMs, select your SIM of choice.
Go into App data usage.
You can see more information here, and even check how much data each app uses.
Check the list of apps and see which are using the internet more. Look for any discrepancies.

This is interesting. I should check on that.

Yes you should. Also, delete apps that looks suspicious.