The desire to screen your references is understandable and some people would advise that you ask people outright whether they can give you a good reference, but my opinion is that you should not. Some people are uncomfortable and will not answer accurately. More importantly, there is no such thing as a “good” reference, just an honest and unbiased reference from a well-chosen referee. Until the referee receives the questions, it is possible that there will be one that cannot be answered with unconditional praise.
Choosing your referees is an exercise in looking into yourself and identifying where you have done your best work, not by looking outward at the potential referees, trying to pick the one who seems most agreeable to give a recommendation in your favour. Some professors will come across quite critical and tough with you and others will give you primarily positive comments and encouragement. Do not mistake this for our sum-total impression of you. It is a matter of personal style and the way we each think we can encourage the best performance from you. When faced with the reference letter, form, or phone call, we will all give an honest reference, for better or worse, based on your performance. The choice of who to ask should be based on who has seen you at your best, not your perception of who seems to be most impressed by you.
That said, it may be worthwhile before you reach the stage of applying for certain jobs or grad programs that you have a conversation with professors to get their impression of whether you would be suited to graduate school or career X, Y or Z.
If a professor does not outright encourage that particular pathway, or suggests other options to you, you might infer that the professor would rate you lower than others in one or more key attributes, and hence an honest reference would reflect that.
If the professor suggests reading material, informational interviews, or other exploratory exercises, you should follow these recommendations and come back to discuss your evolving thinking. You may find that you have new insights to what you want to pursue and your professor sounds more enthusiastic about the match between your attributes and the plan.
Your professor’s agreement to be a reference is an agreement to spend the time considering your qualities and completing the requisite letters or forms, or taking the phone calls from the potential employer, and to give an unbiased and honest appraisal of your qualities within the criteria that are asked. A reference is not a recommendation that you are accepted for the grad program or hired for the job, just an appraisal of your abilities.
