Body shape and comb shape
View attachment 4094213the one in front is a silver laced wyandotte. The one in back is SUPPOSED to be a light Brahma but has so much "mud" in her feathers across her back that I suspect she's actually a light Brahma x dark brahma. Note the short back and deep curve neck to tail on the silver laced: classic wyandotte, known for their curves.
View attachment 4094214the one in front here is a partridge Plymouth Rock
A Columbian wyandotte is going to be shaped like the silver laced while a Columbian Rock will be shaped like the partridge. Combs should match the breed shape too.