Here's what Subcool's said about male selection:
"...(t)he recessive drug traits we seek are stashed away in non-vigorous males. Think about the plant in the wild, the natural progression of Cannabis is back to Hemp. The drugs strains we have today are a direct result of human intervention. Robert Clark says it better In the wild, the early males always win the breeding contest. Without proper selection, these early males cause "acclimatization" of the variety, and a decrease in drug quality. This is the "dominant" state of Cannabis. If it were otherwise, why would we need breeders? All you would have to do is let the plants do their own thing and they would become more potent over time, but they don't. The only way drug varieties ever get better is through human intervention in the natural order.
So,
in order to find these recessive traits we choose males based loosely on the following. At time of germination are there any capitate trichomes on the cotyledon leaves? The number and frequency of non-glandular trichomes; this can be a great indicator of future glandular production. The earliest males to show sex get tossed, no ifs ands or buts. Dominant males are useless for drug (misuse of this rule has caused more hermaphrodite strains than the world deserves) Cannabis. If these basics are followed during each male selection process, then this gives us a much better chance of finding the genes we want. This is not meant to be a guide for anyone, just a basic explanation of what a Cannabis Breeder looks for when selecting strains to work with and/or improve upon."
LINK:
https://www.rollitup.org/t/subcools-breeding-better-cannabis.48785/
And here's Ed Rosenthal, via Cannabis Culture:
"Although males don't produce buds, you can learn a lot about them by the way they grow and mature, and by their odor. Look for the odoriferous plants. They are more likely to be potent. Unless a plant has specific novel or unusual traits, choose only the progeny of healthy, vigorous plants. Be merciless in your selection process and don't let sentimentality interfere with your goals.
Look at the plant parts under an 8x or greater magnification loupe to see which has the most glands. Look at the leaves near the flowers to see which has the most glands and note their bulb size. Dry some of these leaves and test them.
You should also consider how the male compliments the female it will be used to pollinate. Will the two plants' styles fight each other or produce a vigorous hybrid? Will the plant you are considering help to get you closer to the plant you are trying to develop?"
LINK:
http://www.cannabisculture.com/articles/3446.html
