How does Muhammad know he was speaking to his god?

The Word 'his god' mentioned in the question is wrong as it was not his god but Allaah.

I would want to ask the questioner that how do you know that you are a human? Your would be obvious, highlighting the arguments regarding the features of a human which he has.

The same is true for this question, Rasoolullaah (saws) knew that to whom he conversed was not from the creations of this world. He was the One Who is not seen by anyone. The things told to Rasoolullaah (saws) were such, which cannot be said by anyone but Allaah. Can anyone of this world bring a verse like that of Qur'an? This challenge is mentioned in the Qur'an.