You find out by the conjugation of the verb. In every language the verbs are conjugated with the different pronouns.
It is particularly difficult for native english speakers because conjugation of verbs in english don't vary much.
I play You (simgular) play He/she/it plays We play You (plural) play They play
As you see, except for rare cases, conjgation in english only change in the 3rd person singular. Let's see in dutch:
Ik speel Je/jij speelt Hij/zij/het speelt Wij spelen Jullie spelen Zij (plural/they) spelen
There is more variantion, right? The conjugation for plural pronouns are similar, in written form, to the infinitive form of the verb.
So, always look at the verb! Is it conjugated zij singular (she) or for zij plural (they)?