Has Buddha attained Nirvana after death? It is said that the Buddha attained Nirvana after his death. But how do we know that it is true?  Are there any physical signs to recognize?
There is no physical signs of that and we can not verify it. Probably only enlightened beings (such as other buddhas) can verify this, so until we are enlightened we can't.

Btw, he also attained Nirvana before his death too. So-called nirvana with remainder.