To test if it works grab a frost enchant and a flame enchant, boot up a damage add-on (Foundry Tactical Combat [FTC] works great), and take a few swipes at an enemy to see the difference in damage. If Flame damage is higher, by a difference of your Flame Talent %, then you have your answer.

That said, I haven't tested it yet myself since beta, but straight from the developers we have been told it works with all sources of Flame damage. It used to only apply to Dragonknight abilities, and was only an increase of 4 to spell power with Flame damage of spells, so I would presume it does what it says it does as the change was made only a month or so ago.