Hello Quicksilver,
Firstly, you should note that modern phones do not have a setting to disable roaming entirely. They do have settings to disable roaming data. However, disabling roaming data does not prevent roaming for voice and messaging. That said, the Fido Roam charge will incur when you use your device to make or receive a call, send a text message or use data while abroad (see here). If your device has not used your Fido services, you shouldn't be charged roaming fees even if it is connected to a foreign network (ie shows roaming).
In order to prevent unintended roaming charges, it is generally recommended to disable data and roaming data. However, you should note that some manufacturers might allow some data usage to occur despite those settings (see here). That's why it is also generally recommended to enable Airplane and/or Flight modes in order to prevent unintended roaming usage. You could then manually enable Wifi when required.
It should be mentioned that merely connecting to Wifi for regular internet usage would not incur roaming charges. However, if your plan and device are supported for Wifi-calling, your device would be connected to the network via their internet gateway (see image here). As such, using Wifi-calling is technically using Fido services. Though, they have allowed certain usage to be free from additional charges. All the messages and calls you receive (from anywhere in the world) will be taken from your plan's voice minutes and messaging limits. As well, all Wifi-calling messages and calls you make to a Canadian phone number while abroad won't incur long-distance or roaming charges (see link above).
Now to answer your question... I don't think there is anyway to track your roaming usage in real-time (see here). As far as I am aware, Fido does not receive your usage in real-time:
For post-paid roaming, when the user first switches their phone on, the visited network checks in real time whether the home network authenticates the customer and authorises its use abroad... ~ taken from here.
That would result in the Welcoming text message we receive when our phones connect to a foreign network.
...But the data records (CDRs) with the details on each SMS and data session are sent to the home operator with some delay (up to several days).. ~ also taken from link above.
If home networks don't receive usage information in real-time, it's not possible for them to provide customers with notification that a roaming session has begun without a delay.
The situation is different with pre-paid services where a real-time exchange is required to prevent the balance of the roaming customers' accounts from becoming overdrawn. That's likely the reason why pre-paid roaming often costs more than post-paid roaming.
Hope this helps 😀
Cheers
