I'm aware that WordPress stores all the post content of a multisite in the same database table and I was just wondering how it internally differentiates post content from one blog to the other?
Posts from one site are not stored in the same table as posts from another site.
Each blog on a multi site gets an ID and that ID is used to generate a separate set of tables for each site in a multisite installation.
So for example site 1 gets:
where as site 2 gets:
and so on: