Ummm.. If you have the machine in front of you go to Apple System Profiler (in your Applications: Utilities folder, or under "More information" when you choose About this Macintosh from the Apple menu) The Memory section will tell you what you have in each socket.
If it is a new Mac you are looking at, then go to the Apple Store online and Configure the model you want, look at the memory to see how the stock configuration is set up.
Most Macs have a single 512, but Powerbook G4 12" have 256 built in and a 256 SODIMM in the single socket.
Thanks
Trevor
CanadaRAM.com