"initial $40" ....means you are on the plan that gives 100MB of data.
100MB is roughly a tenth of a GB which is not much. So yes, this can be used very quickly and is certainly not for the things you would normally use the data for and should be limited to basic web searches, navigation and email. Each 100MB after this is going to be an additional $25 which is why the TravelPass feature is a better value for data but still needs to be conserved for international travel and isn't for streaming movies or even music.
There ARE ways of telling what you are using but it varies depending on the phone. The general rule of thumb is to leave your data turned off if travelling internationally and to only turn on if away from wifi but to turn back off after using for your directions or web search and such...
You are roaming on not only another carrier's network, but also internationally with a foreign sim card which is not cheap-- especially for data. So-- needless to say this is not intended to be a substitute for a dedicated domestic internet connection and if you use it as such, you will pay for it.