It was said that the summoned monsters would have a default behavior of attacking the caster's opponents if they have not received any command from the caster.
I am wondering how would the summoned monsters identify the opponents when they jump into the battlefield. Would they be confused if they have not been given command and just see a group of humans are fighting each other around them? Or would they automatically (and magically) learn who are their opponents and who are their allies?
From the spell description of summon monster I:
It appears where you designate and acts immediately, on your turn. It attacks your opponents to the best of its ability. If you can communicate with the creature, you can direct it not to attack, or to attack particular enemies, or to perform other actions.
It's not explicitly stated, but the creature automatically knows who your opponents are when you cast the spell, without any communication on your part. It's just an effect as part of the spell.
The only things that are certain are what it doesn't do:
In a lot of cases, it’s pretty clear pretty quickly, just by how everyone is facing one another, and something like pointing or whatever is going to be clear to most things you can summon. You could argue that some delay is justified, being dropped into the middle of melee, but if that were the way to they wanted to do it, they’d just have had the summon roll initiative. At the end of the day, they decided that some delay, some times, wasn’t worth the extra complexity, so the game left it out. As the spell description indicates, they just kind of smoothed it over and said “most of the time is now all of the time: you can point out targets well enough, quickly enough, for the summon to act as soon as they get there.”
That said, it is generally recommended for summoners to learn the languages of the things they summon. Anything more complicated than pointing at a target for “get them” is going to be quite painful otherwise.
