If you tip on the "pre tax" amount, does the server know this is what you did? Or do they just think you left a lower "gross total" tip percentage?

Similarly, I am unsure how to calculate the tip when you have a 'corkage fee' on your bill. You shouldn't have to tip on this fee...but do the servers know that you have tipped on the food only portion of the bill? Or do they think you just left them a smaller tip on the total of the check?