get a PH meter and test it.. there's no real way of "guessing".. also PH fluctuates with time if you let nutrient-solution stand for a while, so you would want to test more regularly anyways (especially if growing in hydro)..
best advice: get an EC meter right away as well, so you know how much you feed your plants too.. following labels of nutrient manufacturers can be a bit misleading, as they are usually basing it on RO water, and as well bigger plants in big environments..
I measure my EC and usually end up requiring a maximum of 1/3rd of the recommendations in my growing environtment