The soil has lots of stuff in it, lets call them pH buffers, that can change slowly over time but mostly have a constant effect on pH. As you add water/nutes, which have their own pH, they will mix with the pH buffers in the soil and shift to a pH somewhere between the two. To measure the change, simply measure the pH of runoff when watering. Ideally, if the pH of the runoff is about right then it's safe to assume the pH of the moisture still inside the soil is about right. Although, as your question implies you already know, the pH buffers in the soil will continue to shift things a bit.

Overall, as long as the pH buffers in the soil aren't terribly off, a little pH fluctuation isn't so bad. Different nutrients are taken up in different concnetrations at different pH ranges, so it's really of most benefit to simply get close, within the right range, rather then focus on attaining a single pH every time.