The only true way to test how accurately your ph tester has been calibrated to is to test it with a liquid that you already know the ph of.
7 PH Liquid
For around 10 bucks a little bag of exactly 7 ph liquid can be purchased. All you have to do is open the bag and use your ph tester on it and how far off from 7 determines how accurate the tester is.
PH Testing Strips
Using ph testing strips to determine the ph of any solution before using your tester is also an option. This option is not as accurate because determining ph level is determined based off color of the strip. This will give a general reading but is harder to determine down to a decimal.