The laptop says it on there. Take me for example. I bought my Dell Insprion 1420 laptop like a couple months ago. It's a Core 2 Duo (as seen from my sig below). But it's a Core 2 Duo design based on the Intel Centrino technology for power saving. It has the sticker on the outside box and on the laptop itself that says, "Intel Centrino Duo"--that's it. It didn't show off the Core 2 Duo by labeling it but it will tell you whether it's Centrino or not in the specifications.
Also, note the "T" in front of the processor number. That usually denotes Centrino technology for mobile use. So look for the "T7100", "T7200", "T" anything and it's a Centrino mobile processor. It's a must have for laptops.