WebRestaurant has stainless sheet cake size markers. Mine are plastic and getting old  but they are quick and easy to use. If you do a lot of sheet cakes for showers and such and they want the  cake marked for individual servings this is the way to go.

they are cool — you only get 60 servings though out of a full sheet 18x26? 

I basically get-  88  2 " servings. I  lay one down then overlap the grid  with the 2nd.

 btw we called it scoring

With all the sheet cakes I do, I need one of them. I have just used a long ruler & a toothpick to score. This would be quicker. :))