
Just look at the extra nonce in the coinbase field of the coinbase transaction.
The counter is monotonically incrementing at a constant pace.
Also this can be used to find how many computers/threads where mining at some time (until they get powered-off). Each thread has another monotonically incrementing ExtraNonce variable.
Absolutely undeniable stuff:
We know Hal mined block 78.
We know Satoshi mined blocks 0 and 9.
Given the extraNonce is incremented by the bitcoin client, the upward trends above sergio theorized as being single clients. This is sound.
The extraNonce seems to reset time to time, but I have no idea why or at what occasion.
Perhaps only satoshi's client did this? Maybe everyone else's extraNonces are just pseudorandom?
I'm going to look into this some more.