Here's why:
* It describes a noun: "She is a capable leader." "Capable" describes the noun "leader."
* It can be modified by adverbs: "She is extremely capable." "Extremely" modifies the adjective "capable."
* It can be used predicatively: "The team is capable of winning." "Capable" describes the state of the noun "team."
Let me know if you have any other words you'd like help identifying the part of speech for!