We don't - and, in fact, in an interview just prior to the release of Shadowbringers, Yoshi P mentioned that we might see familiar faces on the First, but that these people are not necessarily shared souls. Rather, that certain trades tend to attract a certain type of person, nothing more.
The fact that these people are similar to counterparts on the First is not an indicator that they share souls. In fact, the one undeniable instance of shared souls we know about (the WoL and Ardbert) would indicate that people with shared souls are NOT similar in appearance. (That is, unless you created your WoL to look just like Ardbert, I suppose.)
Veloran is correct in that Ardbert and the WoL have a lot of similarities, and that they share these similarities with Azem - a penchant for gathering allies, wandering the land, and performing acts of dogoodery. However, the Azem example is the ONLY confirmed example of a shared soul we have on hand at the moment. Anyone else is speculation, not confirmed by any means - and thanks to Yoshi P's interview, outright suspect at best. Mowena is similar to Rowena because her profession tends to attract Rowena-like people, nothing more. Could she be a shared soul? It's not impossible, but it would require an AMAZING set of coincidences to pull off, considering that time moves at a different rate on the First than it does on the Source, and considering that soul fragments can be reborn as any race. We'd need both Mowena's and Rowena's shards to be born into Hyur/Hume bodies at just the right time so that when the WoL winds up traveling to the First they both happen to be the same age.