In NY, it's typical to tip 20%, so I just look on the bill there for a service charge and if there is one, I'll just tip the server 10% more.
I also know that alot of restaurants up here will collect all the waitresses tips and then distribute them evenly amongst the staff (which is essentially what the service charge is trying to accomplish there). Sometimes a tip will be automatically added to a bill here if a large party is being served.