What signals histone methyl transferases to function?
Histone methylation is regulated by a variety of of events, including transcriptional activation and repression, relative location of the histone in the chromatin (pericentric or not, hetero- vs. euchromatin, etc.), X-inactivation, the cell's point in the cell cycle, and others. A variety of signaling pathways can lead to the (in)activation of HMTs. I found this table at cellsignal.com listing known methylation sites, along with literature references. Elsewhere on the site is this graphic, which shows some of the same info in a different way, and also has links to data at PhosphoSitePlus with links to the protein of interest. If you dig around, you can find all sorts of pathway and experimental information. For example, if you look up PRMT1 (human), you get a brief description of the protein and what pathways it's involved in, as well as links to all sorts of other info. On the histone side, a look at Histone H4 (human) allows you to look at individual modified residues (I picked K21-m2) and find all sorts of literature links and information about experiments used to determine its function.

PS: I'm not affiliated with Cell Signaling, I just think they have a ton of useful protein information, kind of like NEB for mol bio stuff...