Honestly the best trick I’ve picked up is to google the cake name while you’re standing there in the tent. Most of the popular ones have youtube videos showing the actual performance. Way better than trusting the box.
Also look at the weight — heavier cakes usually mean more comp inside. Not always true but it’s a decent rule of thumb. And if the store lets you, check the tubes on the bottom. Thin flimsy tubes = cheap filler.