Okay, well, first, yes, there are lists of words you can look at to estimate reading level. They can give you a rough estimate, but it really is a
rough estimate.
When your child regularly reads unfamiliar books at a particular reading level with good accuracy and comprehension, then I think you can say that your child reads at that level. You can't really count books that you've already read to your child, because some kids memorize books very easily. Anyway, you can look up the grade level of books, and find similar books, with
Scholastic's Book Wizard.