Context: 3rd person vs Usted

Context: 3rd person vs Usted
 

Since the 3rd person conjugation and the Usted form are the same how do you usually determine which one it is?

¿Comiste todas las frutas? : This is clear that the person is addressing YOU

Say in a hall full of people someone came up to you and said:

¿Comió todas las frutas? :

Hay dos posibilidades - Ella comió o Usted comió, ¿como puedo saber cual es correcto?

Is it: Did HE/SHE eat all the fruits?
Or: Did YOU (Usted) eat all the fruits?

Do speakers generally say (Ella/Usted) before the verb in this case to provide clarity?

You've left out 'él' as a possible subject pronoun for the third person. ;)

If the subject is unclear in the third person, it is always said.
If someone came up to me in a hall full of people and asked me the question you proposed, I would assume that she/he is talking to me (using formal address). If their body language indicated otherwise, I would assume different.
The speaker should make the meaning clear if there is any chance of misinterpretation.

Yes. And "it" could be another option.

¿Comió todas las frutas?For example, in our Colombian culture if we are talking about someone that is not present in the conversation, we always add the subject; also, we add a "se" after the subject. For example: "¿Ella se comió todas las frutas?" knowing who "ella" is. By the other hand, we could just say: "¿Se comió todas las frutas?" previously knowing the subject, or in most of the cases, we replace the "se" by "sí"= "¿Sí comió todas las frutas?" But this is something more cultural. If you say "¿Se comió todas las frutas?" it's a good sentence.