Great advice from Mountainbikermark..
He's right - the best way is to turn off 3G, then - absolutely - you are sure you're using WiFi. The iPad will, however, always use WiFi in preference to 3G - so you might think his advice was unnecessary - surely the iPad would always use WiFi?? But there's a subtle problem in that, if you leave 3G switched on (which you'd assume would be OK if you're at home, say, with your home WiFi up-and-running - hey, the iPad will use WiFi) and your home WiFi 'goes down' without you noticing, then the iPad will switch seamlessly to 3G without warning you and, unless you notice the WiFi 'fan' icon is missing from the menu bar, you'll be using 3G.
So, better safe than sorry.....
Switch off 3G in the iPad's 'Settings' menu and then rest easy....
Tim