Full Trust Your application can do anything permitted within the process (i.e. logged-in account) can do. If you are running under ASP.NET 2.0, your application typically runs in the ASPNET account. Therefore, if you limit the permissions of the ASPNET account, you can modify what Full Trust can or cannot do.
High Trust The same as Full Trust except your application cannot call into unmanaged code.
Medium Trust The same as High Trust, except the application cannot see the filesystem other than its own application directory.
Low Trust Further restricts the process so that it cannot make out-of-process calls.
Minimal Trust Basically useless. Perhaps you can write a program to compute PI to the 3000th decimal…
You can use the SecurityManager.IsGranted to check for a particular grant set. For example, this will test for full trust ...
In your web.config file...
Source
The new way to check is this: