Generally speaking, when you press Alt, it's supposed to Highlight every interactable object on screen. Other RPGs do this - games like NWN did it first and even use the same key! However... in Larian games, somehow they've messed this up, and it doesn't work. It only highlights some of the things, some of the time, and it seems pretty hit and miss as to what it picks out or doesn't. One of the more staple mods for their previous divinity games has usually been one that gives everything a highlight and tool tip, like they should have. You could try inundating them with bug report for every time you find that doesn't get tool tipped and highlighted when you press alt. It might help them get the message, but that's about all we can realistically do right now.