You should be able to see it switch when it's working - the WiFi logo on the BB's home page will turn from grey to white when it's all working over WiFi - and that includes your push email. WiFi connections are polled (to save power) so it can takle time (maybe a minute or two?) to flip over when WiFi is in range.
You can test this by switching OFF your 2G/3G/xx connection (or take the SIM out I think), and you'll still get mail when WiFi is active