As in, the mark of the crest shows up somewhere on the skin? To make it a bit more of a "thinker" question, imagine that the crest brand location is related to the personality/mannerisms of the character and their attitude involving their crest. I'm saying implanted crests don't show up as brands, but you can work with those if you want.
Two examples:
Lorenz: Chest
He would totally go shirtless if this was the case to flaunt it. He's quite open and proud of his lineage and crest, so it would be somewhere obvious.
Sylvain: Penis or testicles
You laugh, and I admit it is funny, but even this has a reason other than "skirt chaser". He's actually not proud of his crest. He wants to hide it. The only person who will see it if there was a brand there would be someone he loves and trusts enough to be vulnerable and go all the way with. His supports suggest he doesn't actually do hookups, he chickens out.