Where does vB check login

[marklar]

I don't think you know exactly what I mean. global.php is included in most if not all pages so it's definitely down that path. Within global.php other files are included, session.php being one of them & the most likely.

What I'm asking is, I want to put in a line of code which gets run every time vB checks to see if someones logged in, where do I put it?

I mentioned that it is a login script I am putting in because it would need to be put at the last stage of the vB login check after it has gone through all the variable/session/cookie possibilities.