It is unclear what you are asking for. Your title indicates a field whereas your body indicates a variable. There is a g_form method called hasField() which allows you to see if a field is available on a table view. This can be helpful if you want to see if a field you need is available on the form: g_form.hasField('field_name');

When you are on the request form you can still use the same method, but it is unclear why you would if you are designing the form to have the variable you are looking for. Please let us know the business need for having to find a variable and exactly where you are looking for it (request form, requested item table, catalog task table)