Checking if someone is logged in or not in VB

I am using the login controls and Membership.

At the top of my master page, I have a link "Create your account".  If someone is logged in, I want to change this link programmatically to "My account".

What is the VB code to do this in a Master Page?  How do I establish that someone is logged in or not?

Thanking you in anticipation.

 Roger
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