Lab testing a leaf sample from a male tells you what its pollen will pass on chemically. Males make almost no resin, so smell and stem rub only go so far, but a cheap cannabinoid and terpene panel on young leaf tissue reveals whether the plant carries the THC or CBD profile you are after. Which labs accept small leaf samples and what the results look like is covered on https://breederregularseeds.com, and the cost was lower than I assumed. It turned guesswork into a real selection criterion.