karyotype, buccal smear, and FISH
FISH (Fluorescence in situ Hybridization) is perhaps the best, but it is expensive. The cheapeast method and the least reliable is a buccal smear. A buccal smear searches for Barr bodies that are only found in female cells.
Further Reading and Resources:
What is a buccal smear? From
Medline.
The Cytogenetics Gallery, a very complete site including a picture of a 49,XXXXY karyotype, 46, karyotypes, and a very good glossary.
What a karyotype report may look like. 98k PDF file.
A karyotype illustrates the chromosomal makeup of a cell in an individual or species including the number and arrangement and size and structure of the chromosomes. It is a visualization of all of an organism's chromosomes together, each labeled with a different color. A karyotype is the cell's entire chromosome complement, and is useful in reliably identifying chromosome variations.
Picture credit: Genetic Science Learning Center, Department of Human Genetics, University of Utah.