I'd argue you can 'see' the wall if you place something on it, like:
- your hand
- your frontline's hand (or some other body part)
- a ghost's hand
- flour, dust, tar, enemies' blood, coughing syrup, and other things that could stick to the surface
- gecko, spider, and other creatures that wouldn't fall off; probably also your familiar; dhampir and a high level monk should work, too