Are Santoku blades ambidextrous?
Any knife that has a symetrical blade profile should be ambidexterous in use. Santoku and chef knives typically come with a symmetrical profile1, so you should be able to get a decent choice, whichever of the types you finally choose. Whether you chose a V-shaped or double-bevel, a hollow cut or a convex blade is ultimately up to you, your intended uses and your honing skills. Some knifes feature an asymmetrical edge, but as these are often intended for special uses and for left- or righthanded use (depending on the orientation of the sides), a good manufacturer should point this out. 

If in doubt, either ask your sales person or simply look closely at the blade. Another option is laying the blade flat on the table  (or on a wooden block, if the hilt gets in the way) and pressing along the cutting edge. The angle of the sharp edge will "lift" the spine. If it behaves the same way on both sides, you should be fine, whichever is your dominant hand.

And a big thanks to a ESultanik below who reminded me: 
Knife handles are not necessarily designed ambidexterously, even if the blade is. In fact, I suggest "trying on" every knife you are willing to spend more than a few dollars on - it should feel safe and comfortable and fit your hand size and grip well. A handle that doesn't fit your hand is uncomfortable at best, dangerous at worst. 

1 This forum discusses various styles, including pictures.