How do you identify third person objective?

The 'third person' is not the speaker or the person who is spoken to, it is a third person. Examples:

The 'first person' is the speaker: I came from Texas. Texas is home to me.

The 'second person' is the person or persons being spoken to: You can come at six. The order will be ready for you.

The 'third person' is someone other than the speaker or the listener: Bob made the sign. Frank and Franbrought the balloons. They want to surprise Rose. The party is for her.

The 'objective' is a Word that is the object of a sentence or phrase. The 'subjective' is a Word that is the subject of a sentence. Examples of the third person objective:

They want to surprise Rose. The party is for her.

The Words 'Rose' and 'her' are a third person, not part of the conversation, and the objects of the sentences.