Yes, no, maybe.

Visual can tell you SOME things real easily. but you do need to look at both balls and races. Balls and retainers are easily replaceable, thereby not that important.

Is there grease in there?
- generally a good sign
Is it still greasy in consistency, or has it turned gummy?
- Greasy = good sign. Gummy = bad sign

Is the grease still "clean" - or at least uniformly discoloured - or does it look to carry a lot of flakes and particles?
- uniform = good sign. Flakes, particles and dark streaks = bad signs

Is there rust in there?
- rust is a bad sign, but you can't tell HOW bad w/o knowing where the rust is coming from.
Balls, races and retainers can have very different material qualities. A rusty retainer is no big deal.

So a casual visual inspection is an indication, but not proof.
Plenty of ugly looking bearings will clean up real nice and work just fine, perhaps with new balls.
A bearing will have to be really dry and badly pitted to be possible to discard immediately on sight.
You really need access to ALL the surfaces, clean, to be able to pass judgement.

Do a visual first on all surfaces, when clean.
Balls should be smooth and shiny.
Bearing races do not need to be overall shiny.
On a (typical) used item, I'd expect a shiny contact track/ring formed by the balls against a slightly matte surface.
But the track should be smooth - no dimples.
It should also be symmetrically placed, on the same distance from the edge the whole way round.
And there shouldn't be much - if any - recess to the track surface WRT the surrounding surface.

When assembled, the headset should turn smoothly w/o play, binding or any "notchy" feeling.

"Play" is usually tested for by applying the front brake, and then rocking the bike back & forth against the locked wheel. If you hear or feel clunking, there's play.
This is a bit difficult on bikes with suspension forks, as sus forks may have a bit of play and flex in themselves.
A poorly mounted front brake can also give a false indication.

"Turning freely" can be tested by lifting the front wheel off the ground, which should cause the bars to tilt to one side by their own weight.
Possibly, use your hand to carefully move the bars through their arc of motion, checking the smoothness of the movement and how much force it takes.
Sometimes, the resistance from cabling etc is enough to keep the bars from moving entirely on their own.