It helps to have several boas of known sex to practice on.
Males have proportionally longer tails than females. Adult boas are easier to sex than juveniles.
Both males and females have spurs, the remnants of the hindlegs, low on the sides of the body just forward of the vent. Males have obvious spurs with a thick, hooked claw on the end. Females have less obvious spurs that are sometimes invisible because they are withdrawn into a pit. Females spurs are shorter than male spurs, and the claw on the end is usually thin and straight or not present at all. With some practice, this is good for about 90% accuracy. Probing is good for about 95-99% accuracy.
Good luck.
Paul Hollander