There are several methods for defeating illusions such as Disguise Self.

Any creature with access to Truesight automatically detects illusions as false, so the [True Seeing][1] spell does just that.

> **True Seeing:** This spell gives the willing creature you touch the ability to see things as they actually are. For the duration, the creature has Truesight, notices secret doors hidden by magic, and can see into the Ethereal Plane, all out to a range of 120 feet.

Another similar method is to acquire the [Witch Sight][2] Eldritch Invocation for Warlocks:

> **Witch Sight:** You can see the true form of any shapechanger or creature concealed by illusion or transmutation magic while the creature is within 30 feet of you and within line of sight.

You could also use the [Polymorph][3] or [True Polymorph][4] spells to become a creature with Truesight, such as the [Couatl][5]. 

Finally, you could find or create a magic item that grants Truesight, such as the [Gem of Seeing][6], which also grants Truesight.




> **Gem of Seeing:** This gem has 3 Charges. As an action, you can speak the gem's Command Word and expend 1 charge. For the next 10 minutes, you have Truesight out to 120 feet when you peer through the gem.


  [1]: https://www.dndbeyond.com/spells/true-seeing
  [2]: https://roll20.net/compendium/dnd5e/Classes:Warlock/#toc_46
  [3]: https://www.dndbeyond.com/spells/polymorph
  [4]: https://www.dndbeyond.com/spells/true-polymorph
  [5]: https://roll20.net/compendium/dnd5e/Couatl#content
  [6]: https://roll20.net/compendium/dnd5e/Gem%20of%20Seeing#content