The best way to test cron methods in any new controller action. There is no difference between method, which executes by cron and any other observer method.
You can create your own controller (or use any existing controller and temporarily call there this action).
Following solution is not very clean. Purpose of this solution only understand what do you need:
For example there is ProductController.
This controller has many different actions, like viewAction, etc... You can create any new action like
cronAction and put there your call there:
Don't forget to write your existed module, class and method names. After that call in browser