Figuring out sales tax on a previous purchase is simple math. So is determining what the price of an item is that you already purchased before tax was added to it. If you have a calculator handy, you will have that number in no time.
Find out the sales tax percentage in the area you are shopping in. If you don't already know, the information is available on your state's finance website, or you can simply ask a sales representative. Note that some cities tack on an additional sales tax along with the state.
Convert the tax percentage to decimal form. You do this by moving the decimal point two places left. Add a zero if necessary. (i.e. 3.5% is .035).
Use a calculator to multiply the price of the item you purchased by the sales tax in decimal form. For example, if your item cost $31.50 and the tax is .035, then the additional sales tax would be $1.1025 ($31.50 x .035 = $1.1025). Round that number to the nearest hundredth after the decimal point. The tax you will subtract from your item would be $1.10.
Subtract the sales tax number from your item's price. That will be your total cost before tax. The aforementioned example would cost $30.40 ($31.50 - $1.10 = $30.40).