RE: How do I unit test a Dispose method ?

Nope. I did test-drive this design.
The connection instance represents an open connection to a hardware device.
All conn parameters appear as constructor parameters of this type - one of
which is a SecureString password.
Connection(conn params)

The type creates a copy of the same in the ctor and holds on to it for
subsequent API calls to the device

Finally it has a Disconnect method that closes the connection.

My problem is detecting whether disconnect disposed of the 'disposable'
member fields.
Subclassing and overriding Dispose is an option
override void Dispose()
{
   base.Dispose();
   // check if this.Password has been disposed
}

However there isn't any clean way to know this - unless IDisposable
interface had a bool IsDisposed property.
A crude way would be to invoke another member and expect an
ObjectDisposedException (if the type has been well written)