There' no feedback form your pit crew to let you know that the service is done. They stand motionless and the lollypop man doesn't flinch. No message appears either. I have to keep the car in first gear and rev it and when i move... stop is done.... We need to know what's going on in the stop and when it's over.
ISI said the pit-crew functionality is not yet implemented so they will be motionless the whole time. Currently your only indication that the pit-stop is over is the timer.
For now I just watch the countdown timer... it tells you how long your pit is going to take, then when it hits that time you can go, go, go!