it's on all contacts. It's the date their
lifecyclestage gets set to "sales qualified lead". I'm pretty sure it's null if it's never set, so if you look for all contacts with that property, If a contact doesn't have the property set, it won't even show on the list.
Also, there is a
lifecyclestage property, and you can just check there too for all contacts currently listed as "salesqualifiedlead". That api endpoint I last posted will show you all properties available, and is pretty useful for finding out what properties you want to look for.