On any normal domain, there will be an 'A' Record that points to a web server that hosts a website for the domain.

It is possible for several domains to share the same server, on the same IP address. This is commonly done for low traffic web sites, blogs and the like.

A parked domain will by definition have extremely low traffic, and the company that owns the parked domain will also own thousands of others, so logic dictates that they will all be on the same web server and share the same IP address. (seeing as IP addresses are scarce, and web servers expensive).

Therefore, in order to guess if a domain is parked, one way would be to find out how many other domains share the same IP address.