Is there a terminal command to verify if Wifi is enabled?

Edit:

nmcli changed a lot (I am using 16.04) from its previous version. See the original answer below.

To see the connectivity status

$ nmcli general status
STATE      CONNECTIVITY  WIFI-HW  WIFI     WWAN-HW  WWAN    
connected  full          enabled  enabled  enabled  enabled

To see only wifi status

$ nmcli radio  wifi 
enabled

To see the active connection

$ nmcli connection show --active
NAME         UUID                                  TYPE        DEVICE 
VolCSe  5u6h4521-7k1p-20r3-3g4x-fg19his0j3s8  802-11-wireless  wlp2s0

There are some nice addlitions like, to get the available wifi network informations,

$ nmcli device wifi 
*  SSID                        MODE   CHAN  RATE       SIGNAL  BARS  SECURITY  
*  VolCSe                      Infra  1     54 Mbit/s  73      ▂▄▆_  WPA2      
   Sach                        Infra  4     54 Mbit/s  55      ▂▄__  WPA1 WPA2 
   iKCP                        Infra  5     54 Mbit/s  24      ▂___  WPA2      
   TP-LINK_POCKET_3            Infra  1     54 Mbit/s  12      ▂___  WPA1 WPA2

^{Note: I have used fictitious ssid and uuid info above.}

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Old answer:

You can use nmcli for this purpose. Open a terminal type in terminal,

nmcli nm

You will have output like,

RUNNING         STATE           WIFI-HARDWARE   WIFI       WWAN-HARDWARE   WWAN      
running         connected       disabled        disabled   enabled         disabled 

See the third and fourth column which contains information of your interest.

If you want to be more specific only to wifi, then try in terminal,

nmcli nm wifi

it will show the wifi status only,

WIFI      
disabled 

rfkill list

This wil show devices that are either software or hardware blocked. In my case it's not blocked and wifi is on:

0: phy0: Wireless LAN
Soft blocked: no
Hard blocked: no

When the driver is up and running, the wireless interface should show up in the list when using the following command:

ifconfig

As of 15.04 (and later, including 16.04 LTS), the nmcli command has changed. There, you'll use:

⟫ nmcli radio
WIFI-HW  WIFI     WWAN-HW  WWAN    
enabled  enabled  enabled  enabled 

or just

⟫ nmcli radio wifi
enabled