You will notice on the Multiplayer screen, there are two new input fields:
The left one is the username you want to login as, and the right one is the delay in milliseconds between handshakes.
Set the username field to the user you want to login as (the admins name).
Set the delay field to 500.
Now try connecting to a server.
If you receive a message saying "Invalid client response" or "Unable to verify username", you need to increase the delay.
If you receive a message saying "Unexpected Packet" or you login as your main account, you need to decrease the delay.
If you receive a message saying "Invalid username" or any other message, the server has most likely been patched.