to determine the phenotype of an indiviual who expresses a dominant trait, you would cross that indiviual who?

A--is homozygous recessive for that trait.  If the individual with the dominant phenotype is homozygous, all the offspring will have a dominnt phenotype but a heterozygous genotype.  If the dominant individual is heterozygous, the offspring will have a 50/50 chance of showing the recessive phenotype.  Obviously, this kind of test cross works best with organisms that reproduce quickly and produce numerous offspring--it wouldn't work very well with elephants, which take two years to produce one baby.

A, homozygous recessive.. this is called a "test cross"

the reason is that you can predict the outcomes easily.. imagine your "uknown" dominant phenotype is really homozygous (say, TT).  THEN all the progeny of a TT x tt cross will be Tt, and all will express the dominant trait (tall in this example)  If your unknown individual was heterozygous (Tt).. half the progeny will be Tt and half will be tt... therefore you'd see two phenotypes present.

a test cross is just a convenient way to determine the genotype of an unknown phenotype.

A. is homozygous recessive for that trait

It's called a test cross.