Recently I was asked this question in a therapy session and I'll ask it to you. You race upstairs, only to find them both smiling. He folded his arms across his stomach and grinned back at me, thinking, I suppose, that he had stumped me. But the answer is not what he assumed. If I heard a blood-curdling scream, I would bet my bottom dollar that everything was not so fine.
Kids have different screams just like babies have different cries.
Similarly, as the child gets older, a parent should be able to discern a squeal of delight, from a yell out of anger, from a shriek of pain. To ignore a situation and assume that all is well is being an ostrich, sticking your head in the sand, not willing to take the time to investigate the situation. In any event, never assume everything is OK and ignore what you are hearing. Even if the child is smiling when I get there, I would keep a more watchful eye on my children's play. I would also let them know that fighting, if that were what had prompted the yells, would not be tolerated.