I use a prequalification questionnaire that covers access to budget (can they afford me); timeline (are they exploring or ready to hire) and more. It is pretty thorough and over the years has allowed me to score prospects. I don't mind those in the exploration phase because you need people in all phases of the sales pipeline to keep a steady flow. However, I do need to know how to invest my time. Even with your best efforts you will not close every deal, so don't be discouraged!