When you read Of Mice and Men, you can see several places that George and Lennie know each other. When George and Lennie was having a conversation, George said that Lennie’s aunt wanted him to watch after Lennie. That means they know each other from before. Another example is a little later in the same conversation. Lennie wants George to say the story again, i can’t remember what story it was but it proves they have been together before.
Another one is that George always gives Lennie the fault that they’re previous jobs went bad. When they have worked together, they have known each other. George also said Lennie killed all the mice he got with a woman they worked for. That proves they have been quite close. When you see all this, you can tell George and Lennie knows each other from before.