You can access any information stored such as ConnectionStrings,AppSettings and other defined values in web.config by WebConfigurationManager class in System.Web.Security namespace.
Let's say you have defined and authorization section as:
<system.web>
<authorization>
<allow roles="admin,moderator" />
<deny users="?" />
</authorization></system.web>
The section you just created means users who has admin and/or moderator roles can access pages within and deny everyone (anonymous) who attempts to access without login information.
In order to that just call out WebConfigurationManager's GetSection method as
AuthorizationSection auth = WebConfigurationManager.GetSection("system.web/authorization") as AuthorizationSection;
AuthorizationSection class will give you
Rules
collection which is precisely what you're looking for.