Assuming the other end has filled in what Slack calls Display Name (which I interpret as "preferred name", is heavily implied by Slack's UI, and I think most people will too) … then you've addressed them as they prefer. (And if not, they can change it, and it instantly updates!)

I mean otherwise it's like literally any other communication medium, and you'll have to either take a culturally-appropriate respectful best effort guess at a preferred address, or just ask.