For me, I would have the values stored on a list or in a SQL table that would have a column to check against! It can contain a simple bool yes for been here before and no for new user!
When the use logs in it checks up against the list or SQL table, depending on the value will determin wether they see an intro or not!
For me I would go with SQL and a custom stored procedure returning true or false from the column, its faster and not that taxing on resources!
Simple and easy ;).... im sure there are other ways like through AD (active directory) check the flag if "change password at next logon" is checked. If checked then show the intro if its not checked than dont ;)
there you go three ways! i would still go with SQL but AD is a good contender (no need to add an extra value/table/column) and no calls to DB but its easier to develop!