Q:

Punnett Square Help

In fruit flies, red eyes are dominant over white eyes. Show a cross between two white-eye fruit flies.  
My question is...
How do I know if the white-eye fruit flies are homozygous or heterozygous?

A:

Just to add an extended perspective to all of the answers submitted so far. 
The classic white-eye phenotype in Drosophila is associated with a gene, white (or w) that is carried on the X chromosome (females XX, males XY) i.e. it is a sex-linked phenotype. Male white-eyed flies are therefore technically not homozygous, they are hemizygous since they only have one X chromosome and thus one copy of the gene.
There are other white-eyed phenotypes due to segregation of two genes (brown and scarlet) - this is the standard two-factor cross example in many genetics courses. In this case (no sex linkage is involved) a white-eyed fly is homozygous for the recessive allele at both loci.

