How can I test for differences in standard deviation between two populations. 

If you are comparing two groups, do an unpaired t test. As part of the results, Prism does an F test to compare variances (equiv to cmparing SDs).

If you are comparing three or more groups, do one-way ANOVA. As part of the results Prism reports the results of the Bartlett's test which tests whether the variances differ among groups.