They may be dirty, but I doubt they're glazed. Most squeal can be eliminated by putting a little disk brake anti-squeal paste on the pads where they contact the caliper and piston. 99 cents at the local auto parts store, comes in a pack that looks like fast food ketchup.
While you're at the auto parts store buy a can of BraKleen or similar brake cleaning spray solvent. Before you put the calipers back on the bike spray the pad contact surfaces and the rotors with the brake cleaner.
When you remove the pads look at the contact surfaces. Glazed pads will be shiny and smooth. Break the glaze with some coarse steel wool or sandpaper. It shouldn't take more than a couple of swipes to deglaze them. If they are glazed and you clean them be sure to spray them well to get rid of any crud, dust, steel wool, etc. before reinstalling.