It is a nerve racking thing.. an interview for a job you really want. Anyone that says it isn't, I would question how much they want it! There are some really obvious buying signals here, a prospective employer might give away...however also a few others that might make your self doubt turn into positive thoughts post that interview... I think that being shown around the office and environment, promoting the company and discussing the next stage in the process are great signals you have nailed the interview!
Most people walk out of a job interview feeling one of two ways: like they definitely nailed it, or like they completely bombed. If you ever find yourself in the latter group, you'll probably spend the hours and days following the interview over-thinking every response you gave and every gesture you made, wondering how the hiring manager felt about them.