Tensei Shitara Sainou ga Atta Ken ~Isekai Itte mo Doryoku Suru~ - Vol. 6 Ch. 25

[Meridis]

"How can you tell she is a holy magic user?" panel shows exclusively her huge chest "Just a feeling."

So this conversation alone points out that Huge chest = Holy magic or at least a high chance.

The proportions of their bodies are worse when you also consider that Clarice is canonically 11 years old in this chapter.

don't remember if the manga explicitly explains it but having holy magic, aswell as Ellie's golden lion bloodline thing, makes them grow faster. So, the point IS that their height and body proportions despite their age gives it away.