You should be able to see this on the define roles screen (assuming your theme does it right). The default setting for each capability should be hilighted with a different background.
When you are defining a role, you should basically only use Allow to mean yes, and Not set to mean no (so, in other words, never use Prevent or Prohibit unless you really know what you are doing). To really understand why that is the right thing to do, you need to understand How permissions are calculated