Find the wireless network's security key on a Windows computer in the Network and Sharing Center menu and on a Mac computer in the Keychain Access menu, provided the computer has previously connected to the network. The correct method to find the security key on a particular computer depends on whether or not it is currently connected to the wireless network.
On a Mac computer, open Spotlight by holding Command while pressing the space bar; type "keychain" in the Spotlight search bar. Launch Keychain Access, highlight the System keychain on the left-hand side, and then select Passwords. Next, locate the network in question by scrolling through the list or by using the search box. Double click on the network name and click on the check box next to Show Password. You can also access Keychain Access by clicking Finder > Applications > Utilities > Keychain Access.
On a Windows computer, right-click on the Wi-Fi symbol in the system tray and select Open Network and Sharing Center. Next, click on Change Adapter Settings and then right-click on the Wi-Fi adapter and select Status. Click on Wireless Properties and then select the Security tab. Finally, click on the check box next to Show Characters. You can also access the Network and Sharing Center by clicking Start > Settings > Network & Internet > Network and Sharing Center.