How can you tell what genes are recessive or dominant when looking at actual gene setups?

By looking at a gene set up or a punnet square, a gene that's dominant would be represented by a capital letter and recessive traits are represented by lowercase letters for example brown eyes are considered a dominant trait which can be represented as BB for homozygous, and Bb for heterozygous. Since blue eyes are recessive they would be bb.

The recessive traits are usually represented by lowercase letters and the dominant traits are represented by a capital letter.