Again, there is no way to tell what sex a Nepenthes is before the flower spike appears. The offshoot theory won't hold water across the board. As an example, our female thorelii puts out tons of groundshoots, and so does the male sitting next to her.
The reason that you may think this method has merit is because most Neps are male. I'm not sure of the ratio, but it may be 1 female to every 5-10 males. A good female is hard to find.
Michael, trust us, it will only frustrate when your male is blooming and the known female refuses. It does no service to know the sex if the plant is not in bloom. Many times we will have all males in bloom, then two months later only have females in bloom. We beg and plead with the girls to flower with the boys, but they don't obey our demands.
Why does the sex matter if the plant is not in bloom?
Some Nepenthes never bloom, or at least rarely. N. 'Ron Determan' has never bloomed for anyone, and has been in cultivation for almost 20 years. While the female N. 'Splendid Diana' (or splendiana) blooms constantly.
It's a numbers game. It would be great if we could walk through our greenhouse and sex the plants based on growth habit, but that doesn't guarantee an available breeder. The more plants, the greater the chance of something in bloom.