They don't have a place to monitor usage.
I asked them if I could add the SIM to my own EE account, but that doesn't work because the SIM is part of their EE account. Their support recommended using a 3rd party app to monitor usage, but I don't know if that works (maybe on Android, but does iOS allow this? and this assumes we're using a phone/tablet).
On my Android phones I can see how much data I've used (currently at ~250GB), set alerts, etc, so that's what I'll use. Since I don't know how strict EE is about this 600GB limit, I bought the SIM as if they were selling a "600GB plan", not an "unlimited plan". Someone should go after EE for selling "unlimited" with low limits...
Scancom support told me to contact them if I had any issues with the SIM, so if EE decides to do something, maybe they can fix the problem if there's a good reason (eg: SIM is in a router and you can't see the usage)?