Joel's skin is clean, his jacket, pants, boots and hair are too pristine and clean compared to Ellie who has just gone through battle, you can see how dirty bruised and bloody her hands, arms, face and cloths are. Besides bloody you can see how warn and dirty her cloths are. Even without a battle given the lack of amenities of living like they do without a washing machine and shower he should be dirty like Ellie, yet he's so pristine and clean. The dirt and grime on her hands, and arms , and face and cloths wouldn't come from this battle but living in that world. Yet Joel comes in too clean and perfect.
And why is he showing up late to the battle long after Ellie is certain that all the men are dead and has decided to drop her guard and pick up a guitar? Ellie a seasoned combatant would only do that after several minutes of scoping out the area to make sure they all dead, she would know better than to pick up a guitar give away her position without double checking to make sure there is nobody else and they are all dead. So Joel is several minutes late to the battle. I doubt he'd take the outside position and an unseen battle outside while letting her go inside for close quarters combat, he is the stronger of the two he should be getting into the thick of things not this teenage girl. Why if there was a battle where Ellie is putting her life at risk killing a bunch of grown men would Joel saunter in so relaxed? Why doesn't Ellie stop playing guitar to see who is turning the corner when she sees him out of the corner of her eye? How does she know its Joel and not a remaining enemy?