Pat, usually brake cleaning fluid (aerosol can in any parts place) works great. It evaporates quickly, especially if the engine is still warm, so have some very absorbant towels (paper or cloth) at the ready. When you clean the "dirty" spot, you'll find out how dirty the rest of the manifold that looked okay is. If you have some chemical resistant gloves wear them, or precoat your hands with moisturizer or petroleum jelly and then treat them again after the job's done.