The first way is to sort them according to size. Some chromosomes are much longer than others, as you can see in the picture above. Second, the position of the centromere relative to the end of the chromosome is used to help tell chromosomes apart. Metacentric chromosomes have their centromere near the center of the chromosome. Submetacentric chromosomes have slightly off-center centromeres, such that one chromosome arm is longer than the other.
Acrocentric chromosomes have the centromere located very near to one end.
A third way is to stain them with chemical dyes. Staining results in patterns of bands that are characteristic for each chromosome pair. 46 is the normal number of chromosomes. The two XX's indicate the normal composition of sex chromosomes for a female. This activity was developed through the Natural History of Genes, a University of Utah program funded by the Howard Hughes Medical Institute.