I was surprised when I saw Jason had one because I know he said he wasn't into that kind of thing in the past. I think the test would be if Jon Favreau was following him, he's probably real, since Jon was the one tweeting around I Love You Man promo time which is why the subject even came up! And so far, he's not.
ETA: Aly has confirmed @joshradnor isn't Josh. "Heads up: @joshradnor is a fake. Josh does not twitter. Still waiting to hear from Jason."
Updated list:
Confirmed Twitters:
@alydenisof
@David_Henrie (Ted's son)
@lucyyhale (Robin's sister Katie)
@TheMandyMoore (Amy in Wait For It)
@britneyspears (Obvious I think!)
Unconfirmed
@jason_segel (Aly is following him, but sent him a tweet and is now starting to think that neither jasonsegel or jason_segel are really Jason and has called him out to prove her wrong! He's conversing with Russell Brand but there's not necessarily any proof Russell knows it's really Jason either! Aly is on the case, though!)
Definite fakes
@joshradnor Aly
has confirmed it's not really Josh.
@therealNPH
@NeilPHarris (He spells his own name as Neal and Niel in various different parts of the page. Come on now!)
Other
Accounts worth following for news etc that aren't a celeb:
@_himym_
@nphfan (I can pimp myself, right?
)