Somewhere in... probably mid-1998, when Nintendo Power started including these little Pokémon mini-guidebooks with the magazine issues. The games hadn't actually been released yet, and the cards and anime weren't around yet either. So for months, all people had to go by were these little mini-guides, which were kind of hilariously inaccurate sometimes from what I can remember--half the time they forgot to list a species' second type (so all the Normal/Flying types were just listed as Flying and a lot of the Grass/Poisons were listed as just Grass, for example), they used a weird mix of information from the actual American Red/Blue games and Japanese Blue (like getting the Poliwhirl/Jynx trade mixed up with the Haunter/Machoke one from Japanese Blue), and they were still using the beta names for most of them (so the aforementioned Haunter/Machoke trade was actually described as trading a Kung-Foo for a Spectre.) Sometime after this, but before the games actually came out in English, I managed to find a (badly) half-translated ROM of the Japanese version of Pokémon Blue, and I played through that up to around Misty before the Red/Blue release.
I can't remember how I first found out about Pokémon, but I'm assuming it was through the little guides that I mentioned before, since I did buy Nintendo Power every once and a while back then. I know Nintendo Power had monthly issues and the guides (there were at least 4 or 5 of them, maybe 6? I know I never had the full set but I know I had at least the first few of them) were published during the lead-up to the American release of Red/Blue, so they had to have been running them for months before any official Pokémon stuff made its way over here. Wish I still had those things; I have no idea what happened to the ones I used to have years ago, but they seem to have disappeared. >_<