There was a similar thread like this except with Thunderlord's Decree.
http://boards.na.leagueoflegends.com/en/c/gameplay-balance/wBo9AqcY-its-incredibly-difficult-to-notice-thunderlords-decree
Here is a comment from Stacona and others.
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''Thunderlord's Decree is very noticeable - sometimes its bugged on some champions and does not go off at all - but its a GIANT purple lightning that rains down on your opponent when it procs and you deal a big chunk of damage if you are a damage dealer, specifically a mage type''
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''I was playing Kayle in bots to try it out... and every so often a ridiculous amount of HP would drop off whoever I was attacking's health bar. I'm assuming this was Thunderlord's Decree, but it might have also been some combination of Sated, Guinsoos, and BotRK.
If they could make it a bit more obvious I guess that would be fine. Maybe a text splash similar to a crit?''
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Eh, I don't have trouble noticing it, what I DO have problems with is being able to use it frequently enough. ;p''
So Deathfire Touch may be in the same boat as Thunderlord's Decree. You may not notice it when other particles are flying around or it may work, but not show the effect.