there are electronic pH meters - and the alternative pH "strips"
pH strips are slips of paper you soak in the solution and compare the resulting color to a chart. they are sensitive to aging and storage conditions - "fresh" stock is essential to accuracy.
pH meters are easier to use but in the case of jellies/jams, the probe type/style is very important. big blunt probes will not work in a thick gel. look for a thin probe similar to instant read thermometers. get/use calibrations fluids. pay attention to the directions about "keeping the probe clean" and you'll have decent results.