When I bought mine, I simply brought a live CD in the store, and asked to try it on their laptops. I explained them what this is, and that I want to check the compatibility of the different components with Ubuntu. I sometime had to explain what Ubuntu was...... One place told me a flat NO, that I could not try it. I ran out of the store as fast as I could. At the store where I actually bought my portable (in Dec. 2006), I tested that the display was actually working on the internal monitor, at the full resolution, as advertised on the spec sheet, that the wireless adapter was found, and that I could use the card reader, with a card I brought in. I finally tested the sound itself, which was rather easy to do.
Today, you could bring Linux on a USB disk instead, or even an SD card. Also, in 2010, you should expect every sales rep on the floor to be at least know about Linux. Usually, they should understand that starting it from a CD, especially if this is a branded one, not burned at home, there is no harm that will be done on the machine.
NOTE: The fact that some components are not discovered right at boot from a live CD does not mean that Ubuntu will not support it. Chances are that you will find the proper driver for it. You will just have some manual setup to do.
You may also want to note the brand names and models from a store and check on the net, with Google. You should usually find either some test cases or message exchange about it that will give you a good idea about the model you are looking for.