You can't assess the risk that a client can pose to your success and profitability on a project unless you have information beforehand about how they operate. Although there is no substitute for being able to measure a client from previous experiences of working with them, there are a number of ways that you can obtain an amount of information that will help you in estimating what risks a client will pose to any possible project and working relationship.
A client's commitment to a project will often be a defining factor in a successful implementation—no matter how good your project management and technical skills are, your output will always rely on what the client inputs. There's an IT acronym known as GIGO (Garbage In Garbage Out) that accurately describes the issue in that if the client doesn't put the required effort into the project, then they wont get what they want out of the project.
Although it's difficult to see how important a client sees their project...