Simply enough, she's seen other ta'veren before.

Ta'veren of the strength/duration of the three boys are truly exceptional (Rand in particular) but the Wheel can make anyone ta'veren for any time and with any "strength" it requires to accomplish a correction to the Pattern. Some of these corrections are probably relatively minor (the Wheel can also pull at anyone directly; ta'veren are used for macro changes) "'Minor" or "localized" ta'veren probably aren't as rare as the powerful, civilization-changing ones like Rand and even Perrin-Mat. But some specific events involving many people also need to happen locally (the way the Aiel reacted to Shaiel, for example). Siuan may have seen a few of those in the last 20 years, but she doesn't need to have seen many or even any: the Tower knows this talent - they can tell what this glow a sister sees around someone mean, so to know she has the Talent Siuan only needed to see one before (but maybe not even that..it's a well documented talent (some sisters appears to have been ta'veren, like Mabriam en Sheered) so Siuan could have identified she had it when she saw the Rand, then spoke as if she had known about the Talent before (an AS speciality: it increases they mystery and aura of authority to act as if nothing surprises them or to appear to have kept secrets - making others, even sisters, wonder what other secret aces up their sleeve they can bring out). Unless Siuan says straight out when she discovered that Talent, there's no way to know from "seing ta'veren is one of my talents" if she knew before Rand or not. She probably knew, but it's only probable. She wanted Moiraine a bit destabilized and guessing, so it's not totally impossible she wanted to give Moiraine the impression there were things about Siuan she may not know.