What clues tells us that mercutio really does want to fight tybalt?

From the very beginning of the scene, as Benvolio is trying to get Mercutio to go home, Mercutio very clearly is trying to instigate a fight. When Tybalt approaches, all he does is sass him and turn his words around into jokes and puns. He says "And but one word with one of us? couple it with something; make it a word and a blow," this taunting of Tybalt shows how badly he does want to fight.