How do I check if a module is being used by another?
drush pm-info mymodule|grep "Required by"


grep or don't grep is up to you ;)

Beware this does not list fields that require the module. This implies to views that use fields too.

fields: admin/reports/fields

views: admin/reports/fields/views-fields


those will list fields only, so if you have special pagers, formatters or something else that come from a module you will still have to search for them.