You can go to About this Mac as given in screenshot below
A new window pops up which displays information about your Mac which looks like in screenshot below
If your mac has Mac OS X 10.5 or greater then it is a 64-bit operating system.
Your processor information also lets you know ,if your processor is Intel Core 2 Duo, Intel Xeon, and PowerPC G5 then it is 64-bit processors.
And anyways all the recent Macbooks being shipped are 64 bits.