In practice this is almost always easy to determine. For any given day, you log the hours against a client that you did something for the client. "Something" do

In practice this is almost always easy to determine. For any given day, you log the hours against a client that you did something for the client. "Something" doesn't always equal coding - it could be meetings, setting stuff up, responding to e-mails, etc. For any given hour you bill, just ask yourself "If a contractor charged me for this hour of activity, would it make me - as a customer - freak out?" If the answer is yes, don't bill. Also, I will often not bill some hours for clients that have given me a lot of work over the years.