A little step back,
metalibox,
Going back to have look from the cabin does not really count as a qualified check
A visual check of the slats is not only acceptable, but
required from the flight deck before takeoff and landing.
That's why on the 400 you cannot be dispatched if slats 3 and 4 are giving the indication problem, because you can't see them.
(well, if you just open your cockpit windows you can indeed, but not before landing...
)