I don't know whether he is active right now, but I am more or less certain that he will be among the first 25 users registered to Bitcointalk. So let's do our bit of R&D. Here is the list of first 25 members of the Bitcointalk (in the order of oldest members to the latest):
admin
sirius
nandnor
Xunie
madhatter
nanaimogold
SmokeTooMuch
The Madhatter
xuO4k04c6Ng
The Doctor
AgoraMutual
RogerRabbit
1 currency now
dwdollar
NewLibertyStandard
riX
Sabunir
giik
BitcoinFX
Suggester
livingsoulnation
m0mchil
BlueSky
theymos
nphyx
I think you should be looking for the first 50-100 that registered after the day he left.
It would make more sence that he created an account after he left rather than have 2 or more accounts while he was active in the forums.
So if he did register a new account my bet would be definitely someone around that time.
Maybe even a couple of months later or something.
However, I doubt he registered a new account.
He is probably watching the forums as a "guest".
Satoshi did not want his/her identity to be revealed and that's why it's most likely that he did not register another account. I am certain he is not in the first 50-100 accounts after he left. He was last active in December 13, 2010 and he has good reasons not to be active under his name anymore.