May seem like a stupid question but as a non-iphone user I have no idea

How do you tell if youre using a local Wifi connection or your mobile data when youre browsing sites etc on an iPhone ?

I ask as the managers here have just recieved nice new shiny iPhones and we dont want them using a large amount of mobile data

Need to be able to show/tell them how to tell what connection the data is coming from before we start getting $1000 bills for everyone ;)

TIA