Mk3 gti battery keeps going flat ...

[RainVonBee]

It could either be alternator or battery. Simple way to check the alternator is stick a multimeter on the battery and start the engine, the voltage should jump up to 14v  if not, it's a alternator fault

[analogue_rogue]

ignition off.

Remove positive terminal. connect multimeter set to amps between battery + and cable you removed.. take measurement.  Making sure you have a good connection (use croc clips if poss) turn ignition on and watch current draw.

Post readings .......