There is plenty of evidence in this great account of the life of Chris McCandless that could be used to suggest that he was very foolish. You might want to start off, however, by examining the first chapter very carefully. Gallien, who is an "accomplished hunter and woodsman" in Alaska, gives Chris a ride and comments that Chris had an:
...improbably light load for a stay of several months in the back-country, especially so early in the spring.
This causes Gallien to wonder about who actually Chris was. In his time, Gallien has seen a number of foolish young men who want to live out "ill-considered Jack London fantasies." However, when Gallien chats to Chris more, he discounts this, as Chris asks him a number of perceptive questions about his experience and what kind of natural foods he could find in the wilds of Alaska.
Although Gallien revises his opinion of Chris, he is still deeply concerned, as the following quote displays:
Alex admitted that the only food in his pack was a ten-pound bag of rice. His gear seemed exceedingly minimal for the harsh conditions of the interior, which in April still lay buried under the winter snowpack. Alex's cheap leather hiking boots were neither waterproof nor well insulated. His rifle was only .22 calibre, a bore too small to rely on if he expected to kill large animals like moose and caribou, which he would have to eat if he hoped to remain very long in the country. He had no axe, no bug dope, no snowshoes, no compass. The only navigational aid in his possession was a tattered state road map he'd scrounged at a gas station.
In spite of Chris's obvious intelligence, Alaska is still an "unforgiving place" where you are incredibly foolish to take the might and strength of nature for granted. Clearly, although Chris survived for so long by himself, he would not have perished if he had taken more precautions and prepared more effectively for his time in the wild. A map, as Krakauer later reveals, would have shown him how to cross the river that he felt trapped him in the wild.