Cuál es vs. Cuáles
I'm wondering if you're concerned about the pronunciation. "cuál es" sounds exactly like "cuáles". The first one (with the space) means "which is?"; the second one means just "which?" (referring to more than one). j.dylan2 is correct: the equivalent plural phrase to "cuál es" would be "cuáles son". Both of these phrases could be written without the tildes, but that would make them statements rather than questions.
¿Cuál es? - Which is?
¿Cuáles son? - Which are?
In Spanish you always change Cuál to Cuáles if what your taking about is plural.
¿Cuál es el niño con la camisa azul? Which is the boy with the blue shirt?
¿Cuáles son los niños con las camisas azules? Which are the boys with the blue shirts.