NEW How do I determine total number of Active Directory Forests in an enterprise?

[G-ONE]

I heard that theoretically we could port scan global network for TCP 3268 i.e identifying all Global Catalog Domain Controllers(DCs). So my questions are:

1. How Global Catalog servers is correlated with total number of Active Directory Forests in an enterprise and how can I tell number of forests by identifying all Global Catalog servers? Please elaborate this method.
2. Is there any alternative/automated/practical way to answer this question without checking network documentation and AD design documents or interviewing?

Kindly answer with explanation specific to above mentioned questions.