You can not. Depending on the device, if the Wi-Fi adapter is disabled, it can be actually disabled by the electronics, so you cannot read information from it.
From the Android Developers Blog :
Mac Address
It may be possible to get the Mac address from WiFi or Bluetooth devices. We do not recommend using this as a unique identifier. For starters, not all devices have WiFi. In addition, if Wi-Fi is not turned on, the equipment may not report the Mac address.
source share