Before you add the exp to your character, set a variable equal to the characters level. Call it "Chara LV"
Now add the exp
Create a new variable and set it to the character's level again. Call it "Current Chara LV"
Then make a conditional branch that checks if "Current Chara LV" (> greater than) "Chara LV"
This will check to see if the character lv changed after adding exp.
If so, you can make an animation or display a message stating the character leveled up.