Mirror and the scales... its trial and error really, if I were you start off on 2700 calories and every week weight yourself, you only want to be up about a pound a week. Now you are definitly going to put on fat but if you are putting on more than 1 lb per week it maybe more fat than muscle, if your losing or staying the same raise the cals up by 100 till your gaining a little per week...
As for the mirror just keep an eye on yourself... There is a good thread in the nutrition section on how to work out the calories you need to bulk. ill try find it again for you...
Just don't get frustrated if you arent gaining that fast just take your time, Train hard and eat big and CLEAN and youll reach your goals... Remember Body Building is a lifestyle