The chances are extremely high it's either running Marlin or Repetier. It sounds like you don't have your printer connected to a computer. Try hooking it up and you'll be able to use control software such as Pronterface or Cura or Repetier Host (which is different but complementary to the firmware) to control it. You'll then be able to send g code commands directly to the printer to load filament and do other stuff. You can see a list of g code commands and which firmware supports each at the reprap wiki: http://reprap.org/wiki/G-code

If you send M503 to the printer, which will reply with its settings, you should be able to tell whether your printer is running Marlin or Repetier.

Neither Marlin nor Repetier support x3g. What kind of operating parameters are you expecting to send to the printer?