I'm attempting to come up with a solution to the question in the title, but have been unsuccessful thus far.
Some monsters have the Truesight ability which states the following (emphasis mine):
A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.
This seems like it'd be nearly impossible to utilize any kind of magical disguise against this creature, however a means to thwart players' using Truesight would be to rely on additional coverage via the Nondetection spell, which says:
For the duration, you hide a target that you touch from divination magic. The target can be a willing creature or a place or an object no larger than 10 feet in any dimension. The target can't be targeted by any divination magic or perceived through magical scrying sensors.
and Alter Self's Change Shape option, which says:
You transform your appearance. You decide what you look like, including your height, weight, facial features, sound of your voice, hair length, coloration, and distinguishing characteristics, if any. You can make yourself appear as a member of another race, though none of your statistics change.
As written, this spell combination would defeat players taking advantage of True Seeing spells. However, monsters do not follow the same rules as players, so I don't think this would work if the players were to attempt to deceive a monster with the True Sight sense as the ability is not inherently divination magic (to my knowledge).
My question is twofold. Are there any rules which specifically state that the monster True Sight ability is considered a magical divination effect that could be countered by Nondetection? If not, is there another combination of spells that could be used to counter a monsters' True Sight ability for the purposes of effectively passing off a magical disguise?