how can I determine if two genes are located on the same chromosome?

I believe that you would calculate the recombination frequencies.    The recombination frequencies depend on distances between genes on a chromosome.  The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore, the higher the recombination frequency.  To calculate the recombination frequency, you divide the number of recombinant offspring by the total number of offspring, and multiply your answer by 100.

Consider Morgan's experiment.  After performing a test cross (between a gray-bodied normal wing fruit fly heterozygous for both traits, and a black-bodied vestigial wing fruit fly), he obtained the following results:

965 gray-normal
944 black-vestigial
206 gray-vestigial
185 black-normal

Total offspring - 2300

You will notice that gray-vestigial and black-normal represent recombinant offspring, because they do not display either of the parental phenotypes.

Total number of recombinants : 206 + 185 = 391

Recombination frequency : (391/2300) x 100 = 17%

This relatively low recombination frequency indicates that the genes are linked and hence located close to each other on the same chromosome.

I hope this helps!

The previous answer is very correct. The short version is that if two genes are located close together on the same chromosome, they will not assort independently. So, in a test-cross of a double heterozygote, you will not see a 1:1:1:1 distribution of phenotypes. Calculating the recombination frequency will tell you that the two genes are linked.

Now, as a caution, it is possible for two genes to be on the same chromosome, but to be located so far apart that they do assort independently. So, seeing a 1:1:1:1 ratio on a test cross does not absolutely prove that the genes are on different chromosomes...only that they do assort independently.