He'd set up the train that had brought them to the airport. Of course he'd had monitoring equipment on the platform and in the train. Still, he couldn't have been completely certain the three girls at the airport were the Sailor Senshi. At least, not until they'd transformed before his eyes. Once they had though he'd possessed more than enough information to work with.
Naturally, he'd committed the information to memory. Alongside his humiliating defeat, that information had been among his last experiences before being put to sleep by Beryl. He'd ruminated on it almost constantly and repeated it back to himself before he'd slept. So, once he was ready, it was simple to look into public records and learn their names and where they lived.
And since he'd already spent enough time around the Hikawa Shrine he knew he should be wary of getting too close to Rei Hino, suppressing his energy as much as possible so as to avoid alerting her psychic abilities. Equally, his research informed him of just how clever the Mizuno girl was, so he kept his distance from her too.
But Usagi Tsukino? She'd been far easier prey, Jadeite had more careful about avoiding her cat than her. Even so, he never risked talking to her directly or letting her see him. However, monitoring her from afar more than sufficed and he'd soon learned all about her. Where she spent most of her time, what she typically did there and who usually accompanied her.
Once he knew that it'd been simple to make some logical deductions.
The first was that her lover, Mamoru Chiba, was probably that ever-meddlesome Tuxedo Mask. Jadeite had been especially furious to learn he'd survived their encounter at the airport. His death had been the one and only consolation Jadeite had clung onto. He'd been Sailor Moon's first true ally and, from what Jadeite had gathered, that hadn't really changed since their last encounter. Furthermore, he'd confirmed that throughout most of Sailor Moon's career Chiba had been Tsukino's exclusive boyfriend.
He knew women to be fickle creatures at heart, but his observations of the Tsukino girl hadn't painted her as the unfaithful sort. Indeed, to his disgust he'd witnessed her positively simper over Chiba dozens of times. And besides, the idea that two men could find such an annoying girl appealing had smacked him as just farfetched. Thus, he'd felt safe in presuming Chiba to be Tuxedo Mask.
His second major deduction had been that Tsukino tall friend must've been Sailor Jupiter. Thus, her other blonde friend by default must've been Sailor Venus. And since neither of them had ever seen him he could afford to be a little more direct. Posing as a mailman, bumping into them on the street, striking up the odd conversation here and there. He didn't dare ingratiate himself too much to them, but he got what he wanted all the same.
Eventually he'd struck gold, piecing together when and where to be today.
Upon learning this, he couldn't resist mailing Chiba and the Tsukino girl a little something, though he'd been carefully cryptic.
The message had read:
Dear, Usagi and Mamoru
I am dearly looking forward to seeing you both this Saturday. I have no doubt it'll be a day to remember.
Love,
J.