Dark Souls had ways to determine if it was a chest was a mimic or not with visual ques such as watching for the lids of Mimic chests to occasionally open ever so slightly as they breath. 

Perhaps in your case if where to redesign the first area slightly so the player can see both hallways at the same time and make them both look slightly differently, and then maybe put some burns/char marks on the ground or embers/ash wising in the air or some other subtle indications that one is bad. And perhaps make the hallway longer so that the player can't cross it in one life? Other wise they might just dash through tanking the damage and not attempt to go down the other hallway, because they might just think it will burn them as well. 

In your current design you have also primed your players with two message right before the first light encounter "Move Though the dark" and "You never know what hides in the light" and the 't' junction you have set up funnels the play to the fire trap first. So the lesson learned might just  be going into the light is bad.