Victor, if you're using city water, there should be something like a Department of Public Utilities or something to that effect who can supply info on the water. They'll probably have a standard form with monthly analysis on about 50 different items.(posted 8758 days ago)What you want to know about is not only pH; this by itself is almost meaningless. What you really want to know is what pH and how strongly does the water try to stay at that pH. The term is called pH buffering; a highly buffered solution will try to stay at its pH. There is probably an item on the water analysis called "total alkalinity" that gives a good indication of this, but unless you are familiar with this stuff, won't answer your question. Someone from the water department may be able tell you about something significant that's changed (ie, maybe a major shift in the source of water).
Your best way to answer the question directly is probably to mix two (same kind) developers; one with tap water and one with distilled water, then see if they perform differently. Typically, B&W developers don't have much (if any) pH buffering built in, so they can be easily swayed. Color developers (at least C41 and RA4 styles) are usually well buffered so variations in the mix water can't easily push them away from their aims; thus a commercial color lab is probably not affected much.
Feel free to email me if you'd like me to elaborate.