What is the test for determining fair market value of a property?

Moreover, in cases in which evidence as to actual sale prices is the dominant or even the exclusive factor relied upon in making a valuation, nothing in the law or common sense requires the trier to attempt to ascertain what the property in question would have fetched at a sale through a sales effort begun and ended on the critical date. Surely the fair market value of, say, a residence is not measured by the price which the owner could have obtained for it on the very day upon which he first decided to sell. Rather, the measure there, as in the case here, is what "a skillful broker could within a reasonable period have realized." Bull v. Smith, 2 Cir., 119 F.2d 490; Mott v. Commissioner, supra.