Should I seal my granite countertop?
There are a lot of bad fabricators installing granite these days. I have seen everything from edges that are waxed instead of polished and tops that are dyed to make them look like a more expensive stone than the fabricator purchased. If they did a bad job there is no reason to believe any of it is done right. 

Do the water test suggested above. It will tell you if it needs sealing. It likely does. I would also watch the edges. If they start to go dull in a few months then you really got taken. Polished edges will last a lifetime. Waxed edges a few months. Polishing takes about 15 minutes a foot with very expensive tools. Waxing takes seconds with cheap wax. This is only one example of many tricks to pass off bad granite countertops.  

All granite countertops are not created equal. If you purchased from a guy with lawn sign working with only a cell phone as many today do you got what looks like a good price for a countertop but wait until you try and get warranty work. Good luck because if your installer did not know if it was sealed and you are already doing corrective work you are likely to be even more disappointed in the future as the top ages.