First we need to explain the purpose of applying sealer to stone: A sealer is, in essence, particles of a solid matter dissolved in a transport medium or solvent. Once you have applied the sealer to a surface, the sealer will penetrate the stone and the solvent will evaporate, leaving only the solids in the pores of the stone. This in turn will prevent the further absorption of liquids.
The only purpose of a sealer is to prevent staining by preventing absorption. Sealers DO NOT prevent scuff marks, wear and tear or chemical reactions and do not change the physical characteristics of the stone.
Water Absorption Test
To find out if your stone would require sealer, you can do the water absorption test:
Using a sample piece of the stone in question, drip some water on it. Leave the water for a while and then wipe it with a blotting motion.
If there are any dark spots where the water drops were, it means that your stone sample absorbed the liquids and would benefit from the application of a good quality sealer.
Acid Test
Another very important thing to know about your prospective kitchen counter top, is whether it would react to acids or not. As mentioned before, no impregnating sealer would prevent etching (the damage caused when an acid reacts with calcium) Since it is a chemical reaction, it is not dependent on absorption to occur. There is also no DIY fix for an acid etched stone, and if such damage would bother you, the only way to fix it would be to hire a knowledgeable stone restoration professional.
To do the test, you need a fresh lemon or lime, cut in wedges. Leave a wedge or two - cut side down - on the stone overnight. Remove the wedge in the morning and leave the stone to dry. Look at where the wedge lay - if you can see any dullness, a break in the reflection or can actually feel a rough spot, you have an acid etch. The marks, if present, will also be lighter than the stone.