You can't, that's kind of the point in going hidden.
If you're interested in a username and you don't know if it's been inactive long enough, a moderator will be able to check (but probably not tell you when their last activity was).
[23:23:53] <AndChat|700625> Blue eyes ultimate dragon best card
[23:24:29] <AndChat|700625> You know the one with 3 heads
[23:24:39] <~Lightningkid> just like my dick
[11:35:40] <box> Hampa suck
[11:36:21] <hampa> not the first to tell me that today