Dominance is always defined by the phenotype that occurs in a heterozygous individual relative to the phenotypes of the corresponding homozygous parents.  There are several types of dominance. The observed dominance relationships applies only  to a single pair of alleles of a single gene and for the integrative level of organization being studied.

A dominance relationship may change according to the level of organization of the phenotype. For example, a particular allele might be completely dominant at the organism level (for a phenotype such such as eye color) but be codominant at the biochemical level (e.g., heterozygote has both forms of a gene involved in pigment formation).

Known Heterozygotes.

In order to study dominance, an experimenter must have a group of individuals all of which have the same heterozygous genotype.  These individuals might be  F₁ and reciprocal F₁ progeny. However, in the case of sex-linkage,  only one of the sexes will be used in the analyses (see below).

Determining the heterozygote - Autosomal Genes: For an autosomal trait any individual from the F₁ and the reciprocal F₁ generations (male or female) will be heterozygous. 

Determining the heterozygote - Sex Linked Genes 1: For a sex-linked trait from organisms with either an  XX-XO or a XX-XY sex determination system , the females from either the F₁ generation or the reciprocal F₁ generation are heterozygous.  The males are hemizygous and cannot be used to determine dominance.

Determining the heterozygote - Sex Linked Genes 2: For a sex-linked trait from organisms with a ZZ-ZW sex determination system, the males from either the F₁ generation or the reciprocal F₁ generation are heterozygous. The females are hemizygous and cannot be used to determine dominance.

Review the terms homozygous, heterozyogus and hemizygous