After rewashing, cut the end of the tuber. Any brown or rusty colored area in the middle or part of the way out indicates rot. If any of these signs are present, cut away toward the crown to see whether the tuber has a clear section that includes the crown. If so, the tuber should be viable. If not, throw it away.
Some tubers have insect holes part way down from the crown.
Insect holes are only a problem if they make room for an organism that will promote rot. One may use the Captan or sulphur dry or follow directions to mix any of the chemicals in water and dip the roots. No one expresses any concern about spreading virus by using the dip for successive clumps. When using a liquid dip, place the tubers from one clump in the solution for about 15 minutes. Never dry tubers on concrete, because it tends to draw the moisture from tubers and make them shrivel.