26 C.F.R. § 1.164-5

For taxable years beginning before January 1, 1964, any amount representing a State or local sales tax paid by a consumer of services or tangible personal property is deductible by such consumer as a tax, provided it is separately stated and not paid in connection with his trade or business. For taxable years beginning after December 31, 1963, only the amount of any separately stated State and local general sales tax (as defined in paragraph (g) of § 1.164-3) and tax on the sale of gasoline, diesel fuel or other motor fuel paid by the consumer (other than in connection with his trade or business) is deductible by the consumer as tax. The fact that, under the law imposing it, the incidence of such State or local tax does not fall on the consumer is immaterial. The requirement that the amount of tax must be separately stated will be deemed complied with where it clearly appears that at the time of sale to the consumer, the tax was added to the sales price and collected or charged as a separate item. It is not necessary, for the purpose of this section, that the consumer be furnished with a sales slip, bill, invoice, or other statement on which the tax is separately stated. For example, where the law imposing the State or local tax for which the taxpayer seeks a deduction contains a prohibition against the seller absorbing the tax, or a provision requiring a posted notice stating that the tax will be added to the quoted price, or a requirement that the tax be separately shown in advertisements or separately stated on all bills and invoices, it is presumed that the amount of the State or local tax was separately stated at the time paid by the consumer; except that such presumption shall have no application to a tax on the sale of gasoline, diesel fuel or other motor fuel imposed upon a wholesaler unless such provisions of law apply with respect to both the sale at wholesale and the sale at retail.