Determining beginnerness in NetHack
Solution 1:
You gain four points for each experience point you gain from killing monsters. Once you reach 500XP (250XP for a wizard), you can be certain that you've left beginner status, assuming you haven't gained or lost XP or levels by methods other than killing monsters (consulting the oracle, quaffing a potion of gain level, etc).
Once you've reached level 7 (level 6 for a wizard) you will have almost certainly gained enough XP to reach 2000 score points (1000 score points for a wizard).