How do you get glue off a hard knot wood?

It depends where the area that you are wanting to get at, is but a scraper is usually the most common way. A scraper is a piece of hardened steel, usually about half the size of a standard letter envelope and the edges have been filed in a way so that there is a bur on the edge that can be used to scrape off glue etc from timber. You will obviously scrape off layers of timber at the same time but if it's done properly you won't be able to tell (i.e. don't remove too much otherwise you'll leave a dent in the timber where you've worked). I don't know of any way to remove glue from wood grain without removing some of the timber itself (I'm a fine furniture maker). The scraper is held upright on the flat surface of wood and pulled towards you (you might need to look up pictures on the net). Scrapers can be bought at most woodworking/tool shops. All the best!