There are two ways to keep an eye on your data: the accurate supplier balance on your order page, and the rough counter built into your phone.
Most accurate: your order page
Open the order link from your confirmation email. It shows your remaining data synced from the network, plus the option to top up.
Quick estimate: phone settings
On iPhone, Settings → Cellular shows a per-line data counter (reset it at the start of your trip for a clean number). Android shows similar usage under SIMs / Data usage.