alt-tab will work, but there are 2 problems with that method. The first is that you are going to see temps that are cooler than they were just mere seconds before you alt-tabbed. My 6600gt with the Zalman cooler drops by 5c (sometimes more) within seconds of closing a game.
The other problem is that even if it were reporting maximum temperatures, they would only be the maximums for that particular moment. If you look at a graph of GPU temps over time of playing a game, they are nowhere near stable. At some points the card can be running quite a bit warmer than others. On my 6600gt for example, in Oblivion there are times when the GPU reaches 70c, but there are also times when it is down around 60c. That's a pretty big difference.
Not trying to knock you personally apoppin, its just that if he wants to see the highest temp his card is reaching, then alt-tab just isn't going to cut it.