Depends on the tile, and the thickness of the bits of grout. If the bits of grout are chunky, using a scraper (preferably a flexible thin one made of stainless steel that you buy from bunnings for 5 bucks) is the way to go, but test the scraper in a non obvious area in case it leaves marks (as mild steel scrapers tend to do)
If the grout is really thin/extensive and otherwise an impossible mission to remove because of, say, your tiles being porous, then you're looking at an acid wash to clean them up.
The type of grout is important as well - white grouts tend to be plaster based, soft, and removable with your fingernail/a bit of scrubbing. Dark grouts tend to be cement based and hard as hell.
Whatever you do, take your time - I've seen a few tile jobs ruined by an acid wash when the builder was too lazy to spend a long time and serious elbow grease with a scraper or white scourer.