Sie haben meine Brieftasche.
This sentence has 2 different meanings -
They have my wallet.
You have my wallet.
How can I know which is intended?
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In that sentence it is not possible to differentiate the meaning of Sie. You need either a context or the possibility to see if Sie is capitalized because of the punctuation, or because of its meaning as "formal you". For instance:
— Haben Sie meine Brieftasche gesehen? Ich habe sie verloren.
— Möglichkeit A: Ja, sie haben Ihre Brieftasche. (they-meaning)
— Möglichkeit B: Ja, Sie haben Ihre Brieftasche in den Händen. ("Formal you"-meaning; ok, it happens sometimes to me).
— Haben Sie meinen Bleistift gesehen?
— Möglichkeit A: Ja, sie haben Ihren Bleistift. (they-meaning)
— Möglichkeit B: Ja, Sie haben Ihren Bleistift hinterm Ohr. ("Formal you"-meaning).
That depends on the context. If you talk to someone (single person or sometimes multiple persons, but the informal "ihr" is more common here) it means "you", if you are talking about some people (multiple persons) it means "they".
If you say "They have my wallet." in English, then you can also say "Die haben meine Brieftasche." And then it is obvious again. Otherwise you need to go by context.
If no context is given, you have to ask whom do you mean? Answer: "The ... took my wallet."