Describe an easy technique to judge whether the correct pronoun is being used as a compound subject

Question

triciaal · Answer

compound subject   at least 2 subjects  

not sure if this is an easy technique 

find the pronoun of the sentence and split into nouns
an example 
We went to the mall. Pronoun we. We means "I" and "others". 
My sister and I went to the mall. 
My sister went to the mall.
I went to the mall.

They went to the mall.
They  2 groups of people.
My sister and her friends went to the mall.
My sister went to the mall.
Her friends went to the mall.

compound subject pronouns   we, you plural as a group, they

triciaal · Answer

there are other pronouns but I think by compound subject you are using the plural form

eg  these, those   refer to groups of things 
eg  ours, theirs  belonging to groups of people