To find your FiveM license, first get your IP and port. This can be found in your welcome email or within the control panel at control.low.ms.
Next, join your FiveM server in-game and do not leave.
Minimize and open up a browser. Go to http://playerlist.net/#/view/<IP>:<Port> (replace <IP> with your server ip and <Port> with your servers port), you will see your license against your player.