Disguise Kit & Being Disguised

IAW Player's Handbook:
This pouch of cosmetics, hair dye, and small props lets you create disguises that change your physical Appearance. Proficiency with this kit lets you add your Proficiency Bonus to any Ability Checks you make to create a visual disguise.

Like the Healer's Kit, this will also have 10 Charges. The creature under the effects are 'disguised' for 24 hours.

Creature attempting to see past the disguise must beat the Deception Roll made by the Player/NPC who applied the disguise with their own Insight Check.

Creatures will only attempt to see past a disguise if the disguised Creature's actions warrant suspicion (using magic, committing a crime, failed a Deception Check for bluffing, etc.). Unique mannerisms may also prompt a creature to attempt an Insight Check (i.e. nervous ticks, lisps in speech, etc.)

Environmental effects such as heavy rain could possibly begin to wash away cosmetic applications to a disguised creature, so be mindful of exactly how the creature is being disguised. Some creatures may be provided advantage to Insight if they know the disguised creature well, such as a close friend or family member. Be aware that the disguise is visual - though a creature may attempt to hide their true voice or scent, it is done as a separate Deception/Insight Check unless under another special circumstance.