A good truffle, black or white, should be firm yet resilient to the touch, and should be very pungent—the white variety especially. If a truffle does not smell intensely, don't buy it—and never buy one that feels spongy when pressed with the finger, for that means it's old. Fresh black truffles have a black exterior with a slightly rough texture that suggests a dog's nose. Inside, they should be blackish-brown, with grayish-brown veins. White truffles are a pale yellowish-beige on the outside and brown with beige veins inside.
The most prized (and thus expensive) truffles are usually round and uniformly shaped, but this is only an aesthetic conceit. As long as a truffle isn't broken and has no holes, its shape is unimportant.
Very good fresh truffles can be found in America, but unless you trust the purveyor you are dealing with, it is probably safer to order directly from a good mail-order source. In this country, truffles are sold cleaned and brushed free of all dirt. If you buy a dirt-covered truffle in Europe, simply clean it carefully with a soft brush.