Why would I want to know?
The problem with having very low alkalinity water seems to be that water board will "dose" the water supplied with pH raising chemicals (to protect pipes from acidic erosion). And either the dosing is far from accurate, or the dosing is variably consumed 'tween source and tap, or the dosing is (without warning) changed, etc., etc. I can tell it's happening because the pH bounces about between 7.0 and 8.5 (the only time the pH of the tap water yields useful information!)? The changes make it impossible to predict mash pH when making light-coloured beers (pale ales, etc) ... not that it seems to make much difference? I've already said I might mash at pH5.0 with no noticeable effect.
(I used to have fairly predictable mash outcomes, but since they installed "new" dosing equipment mash pH outcomes are all over the place, usually on the low side).