See also § 1.274-5T(e) relating to the substantiation of business use of employer-provided listed property and § 1.274-6T for special rules for substantiating the business/investment use of certain types of listed property.
If such element is the description of a gift, or the cost or amount, time, place, or date of an expenditure or use, the corrobative evidence shall be direct evidence, such as a statement in writing or the oral testimony of persons entertained or other witnesses setting forth detailed information about such element, or the documentary evidence described in paragraph (c)(2) of this section. If such element is either the business relationship to the taxpayer of persons entertained, or the business purpose of an expenditure, the corrobative evidence may be circumstantial evidence.
If a taxpayer fails to maintain adequate records concerning a facility which is likely to serve the personal purposes of the taxpayer, it shall be presumed that the use of such facility was primarily personal.
26 C.F.R. §1.274-5T