YOU SAID:

how do you know this isn't my first time to party

INTO JAPANESE

これが私の初めてのパーティーではないことをどうお知りになりますか

BACK INTO ENGLISH

How will you know that this is not my first party

INTO JAPANESE

これが私の最初のパーティーではないことをどうお知りになりますか

BACK INTO ENGLISH

How will you know that this is not my first party