Adding vbulletin login check to an external page

[expatdirectory]

HI,
I know this is possible but I cannot remember how I did it last time and lost the reference.

I have a form that I only want to allow my forum users access to. Basically I want to put the login check in the header so:

if logged in: I can get the user name to add with the form
if not logged in: will display the normal you need to register message.

Anyone point me in the right direction?

I tried several searches but to no avail, I know the snippets on VB.org as I referenced it a year back, forgot to subscribe to it!