Is the Wifi module working?

[pfeerick]

I have Pine64 running under Debian with a Wifi module installed that came with the board.
However, I can access the network only by plugging a Wifi adapter into one of the USB
connectors.

Without Wifi adapter the terminal command "ip link" shows no Wifi just
a link called "sit0" and another "dummy0". Is one of those the Wifi module?

How can a get the module active and how can I find its MAC address?

Firstly, check that your wifi module is plugged in the right way around - there are more pins on one side than the other, meaning there is a wrong way around and and aright way around.
Which debian distro are you using? 
No, they are not the wifi module - it should have been called wlan0 and wlan1. 
Check your /etc/network/interfaces file - is wlan0 enabled? you can leave wlan1 disabled.