Your references will typically be contacted by phone for further information. It has never failed to amaze me that people willingly supply references from individuals who end up giving negative references. Obviously, they were not aware that the references would be negative. But that is my point—you need to protect yourself in advance to ensure you have the very best references.
How? By checking your own references. Why? Because while many people are unwilling to tell you to your face what they really think about you, they may be very willing to "submarine" you when they get the chance. Or they may simply feel they need to be "overly honest," telling all, both good and bad.
If you have a written letter of recommendation, you are fairly safe (at least you have documentation of the reference). But if all you have been given is verbal approval to serve as a reference, you may want to double-check the reference before giving out the name to others.
Here is how to do it: do not call them yourself. Rather, have a friend call for you. Have him or her explain that he or she is checking a reference on your background and would appreciate a candid opinion of your skills and abilities. If the reference comes up negative, the best thing to do is quietly drop that person from your list. If you confront a negative reference, you run the risk of that person seeking to further influence others. By quietly dropping that reference, you have done your job in containing a potential closet enemy.
Don't assume. Make sure your references are bulletproof.