Simple RAW 3

[Xaroth]

A145 If it said "affected or detected" then yes, a Rakshasa's true form would not be revealed if True Seeing is used at 6th.

BUT, my MM only says "immune to spells of 6th or lower"; and therefore suggest his immunity would not come into play against such magics. Spells like True Seeing and Detect Magic do not affect, or really even interact, with their 'Targets' directly. Instead, they simply pierce/perceive the magics that are in use on a particular creature, and the normal rules apply.

Now if the Rakshasa manages to gain access to Nondetection... then you're likely to never known his true natural form.

Unless, of course, he wants you to.

Strange, OrcPub says it can't be affected or detected. I'm fully aware that OrcPub is not the Monster Manual though, and that OrcPub is - most likely - incorrect, unless they pull that information from elsewhere.

Regardless, could you point me to the rule that states it wouldn't be detected by it? Or is it simply a matter of common sense?