The store bought contraptions aren't totally accurate, however, most are pretty close if used correctly (such as making sure to clean probe after each use). The testing kits with the tablets you add to water (bottled) would probably be more accurate, but each kit usually only does 10 ph so you would either have to google soil test kits and find one that does more or buy multiple ones.
Does your local agricultural extension office not offer FREE ph testing? The one here does, but if a more indepth soil test is needed then it will cost a little for each test, but not much.
Most of these kits come with the test for N-P-K also so if the ph is fine then you could test and see if one of these other areas is too high or low.
