So we never really see what the Soul Stone can do. Thanos seemed to enjoy testing each stone after acquiring it (burning Thor’s face, teleporting everywhere, tricking Gamora, restoring the Mind stone), but we never see him test the soul stone.
The stone itself seems to exert its power on its own by reading Thanos and talking to him through Gamora. But he never uses it in any obvious way.
When Thanos came to claim the stone, Red Skull knew who he was stating that he is cursed to know all who come seeking the stone, and going on to display knowledge of Thanos’ inner workings. Then later when Thanos is talking to Stark, Stark is surprised to find out that’s Thanos seems to know a lot about him, Thanos retorting “you’re not the only one cursed with knowledge.”
The use of that exact phrasing struck me as very strange, and I don’t think it’s an accident that it was the second reference to a curse. I think that the Soul Stone lets Thanos read the soul of anyone he sees, and immediately know who they are and what’s inside them. Similarly he seemed very familiar with Strange.
There are of course other more throw-away reasons he could’ve known Stark that are more mundane but ultimately a long bow to draw. Thanos might’ve heard about him after he blew Thonos’ army to hell with a nuke in Avengers 1 (not sure how since Loki would be the only one there who knew his name, and I don’t think he would’ve spoken with Thanos after that).