The larger number of comparable properties on your list the better. What you need to establish is that your property is out of line with assessments generally in the same taxing district. Showing that it is out of line with two or three neighboring properties may establish nothing except that they are erroneously assessed. Before you use a property as a comparable, at least drive by and look at it, and compare what you see to its listing card. It happens all the time, especially in localities where physical assessments do not occur frequently.
Professional appraisers are, of course, available to determine market value for a fee.
Owners of residential properties probably won't find the cost worthwhile. But owners of larger properties will find that having one or even two professional appraisals in hand is a good idea. If you have recently purchased the property, there may be a recent appraisal done for the mortgage holder that you can get a copy of. If you believe there is an error in your assessment, you should check the appeal procedure. Usually the procedure is described on your tax bill or can be obtained from the assessor's office.