You'll find the code in Navigation.vb.

Essentially you iterate through the PortalSettings.DesktopTabs collection, and check the TabPermissions.

        Private Shared Function IsTabShown(ByVal objTab As TabInfo, ByVal blnAdminMode As Boolean) As Boolean
            'if tab is visible, not deleted, not expired (or admin), and user has permission to see it...
            If objTab.IsVisible AndAlso objTab.IsDeleted = False AndAlso ((objTab.StartDate < Now AndAlso objTab.EndDate > Now) OrElse blnAdminMode) AndAlso PortalSecurity.IsInRoles(objTab.AuthorizedRoles) Then
                Return True
            Else
                Return False
            End If
        End Function