Simple: look the verb. If the verb turns to the 3rd person of singular it means "she" , and if the verb turns to 3rd person of plural it means "they".
For example: Sie trinkt das Wasser . - "trinkT" turns to 3rd person of singular, so "sie" means "she". Sie trinken das Wasser. - "trinkEN" turns to 3rd person of plural,so "sie" means "they".
A note: trinkEN may refers to "Wir"(We) or "sie"(They).Pay attention for the pronouns. ;-D
Like the articles, the possessive determiners (my, your, his, etc.) agree in gender/number (masculine, feminine, neuter OR plural) and case with the noun they refer to. Here, "my" refers to "shoes" (Schuhe), which is:
2) accusative (because it's the direct object and almost all direct objects are in the accusative case)
The plural accusative ending for possessive determiners is -e: meine.
See this table:
It is accusative, and in the plural accusative form is "meine". If it was just one shoe, it would be "meinen" - Sie trägt meinen Schuh. (Schuh is masculine - der Schuh)
1) "Schuhe" (shoes) is plural. The singular would be "Schuh" (shoe).
2) sie trägt = she wears/is wearing vs. Sie tragen = you [formal] wear/are wearing. Normally, the present tense verb for "she" ends in -t and the present tense verb for "you [formal]" ends in -en. That is also the case here.
You [formal] are wearing my shoe. = Sie tragen meinen Schuh.
1) "She wore/was wearing my shoes"
"Sie trug meine Schuhe" or
"Sie hat meine Schuhe getragen"
Standard German doesn't distinguish betwen the progressive (was wearing) and the simple (wore) aspects. Also, the past tense (trug) and the present perfect (hat getragen) are basically interchangeable in German - the present perfect is just more colloquial.
2) "She used to wear my shoes"
you can still use the two options I listed under 1)
to make things clearer, you could add an adverbial, e.g. "Sie hat immer meine Schuhe getragen" (i.e. She always wore my shoes)
there is also a special German form for this construction, which uses the verb "pflegen". It sounds a bit stilted, though: "Sie pflegte meine Schuhe zu tragen" (She used to wear my shoes)