A suggestion I read here a while ago was to drill two holes through your dry fit dominos. If they get stuck (say you don't like sanding them down too much), just slip a nail through the hole to help pull them out. Makes them super easy to notice.
Tried that with my first use of Dominos. After gluing two of them in and failing to notice the nail holes in them, I gave up on it. Now, all my Dominos have a BIG black X on them.
