Which one of these hens laid a big white egg?

[artbykarenehaley]

Either could be a likely candidate. I'd guess if it was big, the larger bird would be my main suspect.

 

[artbykarenehaley]

For instance the black one could be part polish, which lay white eggs. The speckled one as mentioned by others had a white earlobe, a trait many white egg layers carry. Earlobe color can be somewhat unreliable, but it is usually a good indicator.

 

[Tookie]

Try and do a vent check, here's an article.->
https://www.backyardchickens.com/articles/who-is-laying-and-who-is-not-butt-check.73309/