How will I know if my BlackBerry is using Wi-fi?

There are a few indicators:

The name of the Wi-Fi network will be shown next to the Mobile phone network’s name at the top of the home screen
On phones running handheld code v5.0 or earlier, there will be an icon that says “Wi-Fi” in top left hand corner of the screen it will be bright Black and White when you are using Wi-Fi
On phones running handheld code v6.0 or later, there will be an icon that looks like this

in top left hand corner of the screen the BlackBerry Bullets (4 little dots) will be next to it instead of next to the 3G or GPRS or EDGE icon when you are using Wi-Fi