Open the System Admin module in the GUI, and view the PBX Firmware version number in the module's home page:
What do the Version Numbers Mean?
In the example above, FreePBX Distro 10.13.66-2:
10.13.66-2 - The first number 10 represents the Major Track Number
10.13.66-2 - The second number 13 refers to FreePBX 13 GUI
10.13.66-2 - The third number 66 refers to Linux OS 6.6 operating system
10.13.66-2 - The final number 2 is used as the Minor Release Revision of this Major Track Number