There are two distinct approaches:
Assuming users are reaching the landing page when they navigate the website or originating from a search engine, the one possible solutiuon - not the ideal one - is reading whether a cookie exists (domain.com) and, if exists, evaluate whether _mkt_trk is present. It stores the unique Munchkin tracking code for each lead.
If the cookie is present and _mkt_trk has a value you can then "Change a Form's Follow Up Page Dynamically"
The second scenario is known leads clicking email links. The link can pass an URL parameter. The landing page evaluates whether the URL parameter is passed and redirects accordinfly.