u can get a cheap ph meter from e bay for a couple of bucks. i paid £3 for mine but to be honest i dont even use the thing. if ur gaunna use litmus paper then ph ur water before u feed ur plants and then ph the run off. if ur local water is good and doesnt vary much( just test it it a few times over a couple of weeks) then u shouldnt really have any problems, ph fluctuations are more common , and need dealt with quickly, when ur growin in hydro, soil acts as a ph buffer. its all down to ur local water and what type of nutrients ur using. ph ur water before and after u add ur nutes buddy, it'll show up any potential problems and adjust from there.