От блог Дмитрия:

AspNetHostingPermissionLevel GetCurrentTrustLevel() {
foreach (AspNetHostingPermissionLevel trustLevel in
        new AspNetHostingPermissionLevel [] {
            AspNetHostingPermissionLevel.Unrestricted,
            AspNetHostingPermissionLevel.High,
            AspNetHostingPermissionLevel.Medium,
            AspNetHostingPermissionLevel.Low,
            AspNetHostingPermissionLevel.Minimal 
        } ) {
    try {
        new AspNetHostingPermission(trustLevel).Demand();
    }
    catch (System.Security.SecurityException ) {
        continue;
    }

    return trustLevel;
 }

 return AspNetHostingPermissionLevel.None;
}