Later addition: After scanning the site, it appears I'd be doing well to achieve 90 - 95 percent gender accuracy. Assuming 90 percent, this means 47 percent chance of male getting in with a group of only six. Yikes.
Well to be 100% sure you could cath them for eggs. Still the method is very practical and simple to implement. For your scenario, where the LMB already reproduce, I don't see any harm in introducing 1 or 2 males for every batch of 20 intended females.
In any BOW where large LMB are the goal, one seeks predominance of females. The probing method can help one achieve this via selective harvest of males. In your case where a small stocking of F1s could represent a desired annual recruitment, you could remove all fish but these which you would allow to grow.
