How does a recessive allele “show up” to be seen?

Recessive alleles only show their effect if the individual has two copies of the allele.  

Explanation:

Dominant and Recessive Alleles If your allele pair is FF or Ff, you will have freckles. The only way you can escape a freckled face is to inherit the recessive gene from each parent, giving you the ff allele pair. In general, recessive alleles show up in your phenotype only if you have two of them.