Tough to say. What are the identifying physical characteristics of Earth ponies?
Strength, stamina, and a connection to the world.
Applejack has a mighty buck, and Pinkie can run hyper all day and not get tired. Both of them also seem to understand the world intuitively. There's nothing about apples that Applejack doesn't know, and she just knows where and how to kick a tree to make the apples fall just right without damaging the tree. Pinkie is Pinkie. She just breaks causality because it's funny. She understands the world, perhaps a little too well, and acts like a cartoon character, perhaps because she on some level know that it's possible to do so.
So, to identify Earth Pony traits in Celestia, you need to look for either superior strength and stamina or some kind of intuitive understanding of the world. Honestly, I'd say Celestia's sheer size might be her Earth Pony trait. If she was just Pegasus and Unicorn, she'd be regular size but have both wings and horn. But she's not, she's huge. That's where the Earth Pony part comes in.