Here is some code snippet to see if a user belongs to a certain security group. The code below will work with local group as well as AD groups:

Works in ASP.NET:

//Case Insensitive
bool t1 = Page.User.IsInRole("Everyone");



Works Everywhere:

//Case Sensitive
static bool IsInRole(string roleName)
{
   System.Security.Principal.WindowsIdentity id = System.Security.Principal.WindowsIdentity.GetCurrent();
   System.Security.Principal.IdentityReferenceCollection group = id.Groups;
   System.Security.Principal.IdentityReferenceCollection ntNames = group.Translate(typeof(System.Security.Principal.NTAccount));

   foreach (System.Security.Principal.IdentityReference ir in ntNames)
   {
      if (ir.Value.Substring(ir.Value.LastIndexOf(@"\") + 1) == roleName) return true;
   }
   return false;
}