We know every week. We know if we're profitable on every job. As stated, the first step in knowing whether or not your business is profitable is knowing your true cost of doing business. That isn't as simple as it sounds, because you have to deal with two kinds of costs - direct and indirect. If you have the wrong information on your financial records, have it in the wrong place, or if you have neglected to include all the direct and indirect costs associated with operating your company, then you could be misled into thinking you're making a profit when you aren't.
Direct costs are easier to calculate, of course.
Then, when the job is completed, you do a cost analysis to see if your estimate was accurate. Steve Shepherd breaks each job into multiple categories. We target each job. Each category has a target cost. We watch the jobs closely.