On my Moto X, both WiFi and the AT&T mobile network are enabled. If I'm within a WiFi network that my Moto X 'remembers', my phone connects me to that network. Even so, I don't know whether data is being sent or received over the WiFi network or the mobile network.
In general: How does an Android phone choose which network to use for data if both WiFi and AT&T's mobile network are available? In past I've 'presumed' that the phone uses WiFi unless there is no WiFi signal, in which case it uses the mobile network. But I'm not really sure about that.