How do I detect if someone's in first person?

How do I tell if a player is zoomed all the way in?

Check the CurrentCamera Magnitude.

-- Should be in a LocalScript in the PlayerGui.
local Cam = game.Workspace.CurrentCamera 

game:GetService('RunService').Stepped:connect(function() -- Runs 1/30th of a frame. (A loop)
	if (Cam.focus.p - Cam.CoordinateFrame.p).magnitude <= 1 then 
	Print 'player is zoomed in. (Do stuff here)'
	else
	Print 'player is zoomed out.  (Do stuff here)'
	end 
end)