Apparently, yes in some cases.
According to eTrailer, an online merchant of trailer related items:
Trailer axles typically have the bow ... in them to help
compensate for the vertical load they bear and still have proper wheel
camber. This way when the load is put on them the axle will flatten
out and the wheels will be aligned properly.
Square axles are a bit of a wild card though as they aren't as
standard as the round tube axles. I have seen totally straight ones
and I have seen curved ones.
The purpose and effect of this curvature is further elaborated in an Eastern Marine article:
Most trailer axles are designed with a “bend” in the tube that will
deflect when the load (weight of the trailer) is applied. This will
theoretically bring the trailer wheel camber toward a 0 value that is
the best setting for most trailer applications. This will allow the
tires to wear evenly and allow the longest tread life. ...
Both Sprung axles and Torsion axles typically are built with a slight
positive camber but custom axles can be specified with or without
camber. A trailer that runs long distances without a load may be
better without axle camber. An example of this might be a long
distance boat transport trailer.
As to whether the bend would be visible: at minimum if you held a straightedge up to the axle (long level for instance) it should be visible. It may be hard to detect otherwise, particularly because you often can't get a good unobstructed view of the axle.
Just because some axles have deliberate curvature doesn't mean all bends are good! The axles the articles are describing would I think always have either a consistent, gentle curve or at least be symmetrical. This is distinguished from bends from damage which would almost certainly not be symmetrical (left-right) and could show other signs of proximate damage from whatever caused it.
(In my own experience, on the bent axle on my now wrecked utility trailer the bend occurred at a relative weak point - where the wheel / hub spindle met the thicker part of the axle.)