Question: How do I know when my character has been fully developed? How will I know when to stop adding detail before they become a mess of a character? Also how do I add detail to an under developed character?
While there's no definitive answers to these questions, I would suggest you pay close attention to your feelings.
The reason you add details and develop your characters is so both you and your reader can develop strong feelings about them. You may love or hate a character. You may approve or disapprove of them. But you should not be bored by them. You should be fascinated with the character -- how and why they do what they do. You should feel a strong empathetic connection to them.
In the process of developing a character, you want your empathy, fascination, and emotional response to the character growing. If you find those things waning, that's a problem.
You may want to back up a little. Ask yourself if there was a point in the development process when you had strong feelings about the character. Perhaps you went off the rail at some point and need to go back and start over from there. Perhaps you have been adding details that have made the character less interesting or weakened your emotional/empathetic connection to them. Maybe you've fallen back on stereotypes. Maybe in the process of trying to make a character more interesting and original, you've made them less authentic or relatable.
If your character has so many details that they are starting to become "messy," the problem may be that you've been adding details that don't have a lot of significance.
For instance, if you know what the core traits of a character are -- the things that are most significant in terms
of the story, as well as being fascinating and evocative -- then focus on adding details that will support those traits and make them feel authentic. Creating such "telling" details will be more important than adding random details that have no connection to what is essential about the character or the role they will play in the story.
(It may also be that you didn't identify the character's core traits before you started developing them. In that case, go back and figure out why this character is essential to the story and what core traits they need to play this role.)
As for how to add details... for non-POV characters, you will need to look for external things that reveal the characters inner traits. These may include details about what they do, how they behave, how they speak, how they dress, etc. What does a character do to hide or compensate for an aspect of themselves they feel insecure about? How do they try to get their emotional needs met?
A general guideline in writing is to keep in mind the signal-to-noise ratio. You must include all the events, characters, words, etc. needed to fully express the signal (the story you are trying to tell and its thematic messages). But events or words that are not necessary to express the story are noise that should be cut.
You can apply this to character creation as well. Include the details necessary to make the character feel authentic, to fulfill their role in the story, to help you empathize with the character, and that give you a strong emotional response to the character. Anything after that may be just noise.
Or, as Michelangelo might say, remove all the bits of rock that aren't the statue until the statue is all that remains.
Best of luck.