In the app
Open the app and choose your eSIM. It shows the data remaining, the data used, and how long the plan has left to run. This figure comes from the network rather than from your phone, so it is the one to go by.
Refreshing it
Pull to refresh on the eSIM screen. Usage is reported by the network on its own schedule, so the number can lag behind what you have actually just used, particularly right after a large download.
Your phone's own counter
Your phone also counts data per line, in its cellular settings. It is useful for a rough sense between refreshes, but it counts differently from the network and it keeps counting until you reset it manually. Do not use it to decide whether you have run out. See "Why our usage figure differs from your phone's".
Running low
Top up before you hit zero if you are relying on the connection. Buying a top-up needs a connection of its own, which is much easier to do while you still have one. See "Top up a plan that has run out".