Is a Plasmoid’s Shape Self similar to a Changeling’s Mask

A Big Kobold's OK

The Player's Handbook p 222 (disguise self) says:

You cannot change your body type. For example, a human caster could look human, humanoid, or like any other human-shaped bipedal creature.

The example, which is not in itself rules, gives a reasonable answer that body type is 'general shape and number of limbs'. It also calls out humanoid (apparently the type, considering the later clause regarding shape) for humans. A kobold is humanoid, so a changeling could reasonably become a larger than normal kobold (given the height change limitations of plus or minus one foot in the spell).

How It works For Changelings

For the complete limitations of the ability (see the Minor Change Shape sidebar in the link), as it relates to changelings:

• You can [become] 1 foot shorter or taller,

(Simple enough)

• thin, fat, or in between

(Conservation of mass not being a thing in D&D... No real limitation listed; though flesh appears to be required, so no skeletal creatures)

• You cannot change your body type. For example, a [changeling] could look human, humanoid, or like any other human-shaped bipedal creature.

(This is extraordinarily broad; definitely any humanoid, most monstrous humanoids (young minotaurs, medusae, large dromites), plus many fey, several devils and demons, many celestials and a significant variety of other outsiders, some giants (like half giants), some aberrations (like elans), just about any undead, and perhaps even a medium ape or baboon)

• Otherwise, the extent of the apparent change is up to you. You could add or obscure a minor feature or look like an entirely different person. The [ability] does not provide the abilities or mannerisms of the chosen form, [but it alters your actual] tactile (touch) and audible (sound) properties [though not that of your] equipment.

(You have extremely broad leeway, and you look, and feel and sound like the form you've assumed. The sound properties are demonstrated as Rules As Intended at the very least in the Eberron Campaign Setting and Races of Eberron sections regarding changelings, such as RoE p42-4: 'A Day in the Life' section.)

How It works Otherwise

As for the limitations of the disguise self spell itself, the only differences from the above are that the caster only appears to have the listed changes, and its equipment is visually disguised (so no effect on move silently, for example).

Detect Magic will not identify a polymorphed Changeling.

Only spells (or abilities that replicate specific spell effects or invoke a "spell attack"), magical items, and effects specifically clarified as being magical by descriptive text count as capital-M Magic, which would be detected by Detect Magic. Otherwise, even something which would seem supernatural to us is just part of the "background magic" of the world, which is a part of nature and always functions, and is not considered magical as far as game rules are concerned (as per Sage Advice).

The changeling's Shapechanger ability states:

Shapechanger. As an action, you can polymorph into any humanoid of your size that you have seen, or back into your true form. However, your equipment does not change with you. If you die, you revert to your natural appearance.

Contrast with, for instance, the Change Shape ability of certain dragons, which always has wording like:

Change Shape: The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form.

The dragon's ability is expressly magical. The Changeling's ability is not. Therefore, effects that detect magic are no use in identifying a polymorphed changeling, though they would help you spot a polymorphed dragon.

There are, however, other means to identify a polymorphed Changeling.

Any being with Truesight can:

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

The most obvious way to gain Truesight is the use of the 6th-level spell True Seeing.

The spell Moonbeam also has an extra effect when it targets shapechangers, as failing to save against the spell forces them to resume their original form - though this is a damaging spell, so probably not one to use unless you're already on very bad terms with your suspected polymorphee.