Actually, you CAN tell if your pokemon will be male of female in Red/Blue/Yellow, when traded to a Gold/Silver/Crystal. Pokemon genders were assigned based on their attack DVs (IVs, w/e) back then. To keep the two generations compatible, the game designers had to ensure any new features in Gen2 (like gender) could function with information from Gen1. So, they arbitrarily decided the Attack DV (IV) would determine gender. (If you're wondering, another example of strange game mechanics used to keep cross generational compatibility was the determination of both special attack and special defense from ONE single gene - the special gene)
Pokemon species were given gender ratios (for example, most starters have ratios of 87.5% male to 12.5% female) and these ratios determine the range for a pokemon's possible attack gene. In my aforementioned example, a male pokemon would have an attack DV anywhere from 2-15 (remember, DVs only when from 0 to 15 in Gen1 and Gen2) while a female pokemon, of that same species, would have a range from 0 to 1.
A simpler example would be a 50:50 gender ratio. In this situation, a male would have an attack DV range from 8 to 15, while a female would have an attack DV range from 0 to 7.