How do I see what's linked to my Steam account?

There isn't one page for it, it's a few places. Account details shows your email, phone and payment methods. Steam Guard lists the devices that are signed in, and has the button to sign out everywhere else if something looks off. Publisher accounts like EA or Ubisoft are mostly managed from their side. Also look at the API key page. If there's a key you never made, revoke it, that's what a lot of the trade scams leave behind.