The right front seat window on Tippit's car was rolled up or closed. So Oswald must have talked through the little side window vent that cars used to have. It can partially be seen open in the photo below.
It's kind of interesting, to me at least, that Tippit didn't roll down the window to get a full look at Oswald's face first. I would think if he suspected something more about him he would have done that in order to get a full look? And see how the person reacts? That's also Tippit's clipboard inside the window. He didn't write anything down about the stop either.
My guess is that he suspected "something" - Oswald changed walking direction - but I don't think he connected it to the assassination. I simply don't believe he would have checked the suspect out this way if he thought the man was involved in the murder of the president. Yes, a complete hunch on my part.
