When properly cared for, a tuning fork can provide a stable pitch reference with absolute accuracy nearly as good as an electronic tuning device
What kind of things affect the pitch of a tuning fork? The biggest effect is temperature. A good rule of thumb is the pitch of a steel fork drops one cent for every eight degrees rise in temperature Fahrenheit. (That's about five degrees Celsius.) So if you want one half cent accuracy you would need four degree F temperature accuracy. Remember that when you bring your tool box in from the cold car.
Suppose that even at the correct temperature, you still suspect that your fork is not accurate. What can you do about it? Well, you can file or grind it. But before you ruin a perfectly good tuning fork, here are some things you should know about forks.
A good fork has a precise balance between the two tines. That's what gives it such a long sustain time. The two tines vibrate in opposite directions in such a matched fashion that very little energy is lost through the central support. Whatever you do to your fork, be careful to maintain that balance as well as you can. If you file or grind some of the metal from one tine, try to remove exactly the same amount of metal from the other tine. If you don't you may find your fork has become imbalanced and it will have a short sustain time. If you grind or file a notch where the two tines come together, make sure that notch is as close to the center as you can get it. If you notch one side of the fork, it is a good idea to notch the other side as well to maintain symmetry.
If you are considering trimming your fork, then you must have access to an electronic tuning device whose precision you trust. Choose a temperature to be your standard. It does not matter what that temperature is, so long as you can ensure that your fork will be at that temperature when you use it. At that standard temperature, measure the pitch offset of your fork using the electronic device. If the pitch of the fork needs to be raised, then you want to file or grind some metal from the top ends of the fork, so as to make it shorter. If the pitch of the fork needs to be lowered, then you want to put a notch between the tines so as to make them longer.
You will have to find out by trial and error how much filing or grinding it takes to change the pitch by a certain amount. Start off very conservatively. Use just a couple of strokes of a file and then check the pitch of the fork to see if any detectable change has occurred. If there is no change, then try doubling the number of strokes of the file until a change is detected. Keep a record of how many strokes you use and how much the pitch changed as a result. Gradually approach your target pitch, being careful not to overshoot the target. Remember that the act of handling your fork and filing or grinding it can introduce temperature changes. When your fork gets close to the target pitch, let the temperature stabilize to your standard temperature before each time you check the fork's pitch.
If you don't have access to an electronic tuning device but have a Windows computer with a microphone input, then you can get a free tuning program for your computer by downloading the trial version of TuneLab Pro. Make sure to calibrate your installation of TuneLab Pro using NIST tones before you trust it to act as a reference for calibrating tuning forks.