The most accurate way to measure pH is with a pH meter. The only problem with pH meters is that they are not cheap and they require some maintenance, but if you are doing research or have a large area to sample, then a pH meter is the way to go. The pH scale can vary by about 0.2 units from lab to lab and even more than that depending on the pH of the water used.
If you are using a meter that isn't calibrated, it will have an error. You can buy meters that are calibrated for your soil type and the amount of acidity in your area, but even these meters will not be perfect. A good calibration can help you determine the amount of acidity in your soil and also help you understand how to adjust the readings when you dig new holes.