VBoxManage list bridgedifs | grep ^Name

Name:            en1: Wi-Fi (AirPort)
Name:            en0: Ethernet
Name:            p2p0
Name:            en2: Bluetooth PAN

On boot:

[default] Available bridged network interfaces:
1) en1: Wi-Fi (AirPort)
2) en0: Ethernet
3) vmnet1
4) vmnet8
What interface should the network bridge to? 1