Just curious as to why you'd want to do this, that is, if you mean the final product. pH is most important in the mash and to a lesser degree, the boil. The pH of beer is typically in the low 4s. It is brought down to this pH through the mash, boil, and fermentation.
You could test it with a meter (expensive) or the pH strips made for the pH ranges specific to brewing. You can get them at any LHBS and they're accurate enough when you consider the expense and calibration of a pH meter.