The AcuRite digital thermometer w/ a probe is the way to go.
(The one MBrian posted would also be fine.)
Here's the AcuRite.
It shows two temps; the temp of the actual box, and the temp of the probe. Place the box on the cool side and the probe on the basking spot, and you're set. It also reads humidity, which should be 20-40% in a beardie enclosure. $12 at Walmart.
If you are willing to spend a bit more money, I highly recommend a tempgun.
http://www.tempgun.com/main.html
Point it at something (from up close), press the button, and you get a temp reading. I have a PE-1 and I love it; I use it for measure the temps in Budo's enclosure, the snakes' enclosures, Budo's
bath water, and the outdoors, when I'm thinking about whether or not to take Budo outside for a while.