As I noted in a prior post, the county's assessment of your property (which you can find on your tax bill) should be a percentage of its actual value. The percentage is determined by the common level ratio for the county in which the property is located. The common level ratios for Pennsylvania counties can be found by clicking here.
An ideal time to evaluate your assessment is immediately after you purchase a new home or other property. If the purchase was an arms-length transaction, the price you paid is probably the actual value of that property, or very close to it. If you multiply the assessed value by the applicable common level ratio (percentage), you should end up with a number around the purchase price.
If you multiply the assessed value by the common level ratio and get a result that is significantly higher than your purchase price, the property is likely over-assessed, which means that you are probably paying too much in real estate taxes. You should thus consider filing a tax appeal.
For example, let's say you just bought a house in Chester County for $520,000 that is assessed at $400,000. The present common level ratio for Chester County is 1.70. When the assessment of $400,000 is multiplied by 1.70, it yields a value $680,000 - far more than the $520,000 that you paid. The property would thus be overassessed.
Note that if you are appealing your annual assessment (in other words, if you are challenging an assessment that has been in place for a while, rather than one that was set following renovations or new construction), your appeal applies to the following calendar year. Thus, in 2012, you could appeal your assessment for the 2013 tax year.
If you think your property is overassessed, you may wish to contact an attorney experienced in handling real estate tax appeals. You always want to make certain that you are careful about filing an appeal since it opens up the assessment of the property to scrutiny and, in a worst case scenario, could result in your assessment (and thus your taxes) being increased.