There is no proof, except I believe Gavin himself in an interview said that btc was running for a year in a private network with just a few people involved, so we are left with conjecture and innuendo. I believe he said this on that web show he did, or maybe it was that presentation.
You can see the historical hash rate/difficulty here: http://bitcoin.sipa.be/speed-ever.png
Let's assume that satoshi was the only one generating between 2009-01-03 (the genesis block) and ~2009-12-30 (first difficulty increase), which I think is doubtful considering that he announced Bitcoin to the world 8 days after genesis. Let's also assume that he generated 1 block every 10 minutes, which as you can see from the dip in hash rate isn't the case.
That means he generated 51984 blocks, or 2,599,200 BTC, which is about 12% of the 21 million total.
Keep in mind that this scenario is very unlikely, I'm fairly certain there were others mining during that time period, and that a block was not created every 10 minutes during this period.