Sometimes the surfaces can be a little matte and dull, whereas they should be glossy and dotted with iron, so it's quite an easy thing to see. That being said, if you're using different clays and glazes it may be harder to gauge what needs refiring.
Reduction occurs when you reduce air intake during the firing. This can produce more of a stoneware look, some glazes may change color. Be careful not to use reduction for an extended period of time or it will bend your shelves, melt your kiln brick and burnout your pots from the inside out. Reduction is required for red glazes. The only reason to use this method in the video is to make the clay body darker.
