CPU-Z will also tell you which slots are populated and with what kind of memory (manufacturer, part numbers, speeds, timings, date of manufacture, etc).
You'll avoid potentially hard-to-track-down errors/BSODs if you ensure that you populate your notebook with sticks from the same manufacturer and of the same part #. DDR3 at speeds 1333MHz and above is notoriously picky about how well it plays with other sticks. Just read some of the Corsair forums.
What I did was open up my laptop (G74) and take note of what sticks were in there (in my case, ASUS used 2x4GB of these:
http://www.superbiiz.com/detail.php?name=D3-13S4GS). So I bought two more identical ones, and all 16GB work well together.
Not saying that you can't mix them up, but its risky that you'll end up being stuck with DDR3 sticks that don't play well together.
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G74SX-CST1-CBIL, i7 2630QM 2GHz
32GB DDR3 RAM @1333MHz
GTX560M 3GB DDR5 (192 bit)
17.3" LED 1920x1080
Sentelic TP, BIOS 203
Debian Linux Wheezy (Testing) Kernel 3.2, NVIDIA 295.40