Sometimes you can tell by the vent, the females will typically have a more elliptical or oval shape while males are round, however this is completely unreliable. The only other two ways of checking sex is after spawn to squeeze the belly gently with two fingers, either you will get a white substance called milt from a male or greenish eggs from a female from the anal vent and this is also only able to be done for roughly one month of the year. The only other way is like Dream said; males are typically smaller than females. It is said that almost all LMB over 8 or 9 lbs are female.