Given a downloaded PDF of a publication, is there any way to detect if it's a journal or conference paper?
Just search for the title of the paper as a phrase, online. That will almost always lead you either to the journal, or to the conference.

If that doesn't work, go through the list of authors one at a time, and find their university home pages, if they exist; from there, look for their list of publications: that will often have the full citation, and that will let you know whether they are journal or conference papers.

You could also try searching on one or more of the online reference compilers, such as CiteULike, Sparrho, Zotero, Mendeley, or even serial spammers ResearchGate

There are also existing tools such as Mendeley Desktop that will try to find a pdf's metadata for you, with varying degrees of success.