I am about to start a new campaign. One of the characters backstories is that their family was slain by fiends, they therefore have sworn to get vengeance on all fiends. This character is a Ranger with favoured enemy fiends. 
another character is a Paladin, who has the ability to detect fiends.

A third character is a chaotic good Tiefling, who goes about hooded to hide their nature.  

is a Tiefling by nature a fiend, and hence detectable as such by the Paladin? Is a Tiefling subject to the effects of spells that target fiends, and the advantages a Ranger with favoured enemy - Fiends has. I am happy for the character who hates fiends to play out their innate distrust of the Tiefling, but wondering how to best portray societies worldview. 

Technically? No.

If you want to play it that way? Sure, why not.

Ultimately, a tiefling has fiendish bloodlines but is not in and of themselves fiendish.

Tieflings are classed as humanoids, not fiends (and technically it appears to be a bloodline curse, not even having any fiendish blood).

Note that being classed as a fiend would also mean the character would be immune to spells that target humanoids, such as Hold Person.

All Tieflings have an ancestor somewhere up their family tree that is a full Fiend, but they are not themselves Fiends.  In a lot of DnD cultures, the resemblance to their Fiendish ancestor makes it really easy for other races to mistrust them, but there's nothing inherently evil about them.

ahhh, this was exactly what I was just wondering ad my cleric did a detect evil qnd good. if the teifling in the party would ping. it also answers if he changed his direction and decided to given in to fiendish like traits if the cleric would notice a change. so basically, it sounds like no different than if a good person decides to become a murderer. 

It wouldn't entirely surprise me if dual creature types (as per the current UA) made its way into tieflings or some variant at some point, but currently they're just classed as humanoid.

Quote from Pantagruel666 >>

It wouldn't entirely surprise me if dual creature types (as per the current UA) made its way into tieflings or some variant at some point, but currently they're just classed as humanoid.

 I sincerely hope that doesn't happen, since all that would do is given them a few new vulnerabilities without granting any new perks to offset them.

Quote from 6thLyranGuard >>

Quote from Pantagruel666 >>

It wouldn't entirely surprise me if dual creature types (as per the current UA) made its way into tieflings or some variant at some point, but currently they're just classed as humanoid.

 I sincerely hope that doesn't happen, since all that would do is given them a few new vulnerabilities without granting any new perks to offset them.

I was also mulling Tieflings, and think those Feywild Elves would be even more in line for this, Dragonborn too.  Frankly, and I said this in one of the threads, I think it might have been more daring to do this "dual nature" introduction by using Dragonborn and Draconian (whose 5e existence is in at least draft form per celebrity game evangelist), and/or Eladrin/Shadar-Kai, Warforged, and maybe Tieflings as a way to show players how this innovation could really play out.  I think WOTC may very well have those ideas on the board, but opted to introduce the lineage "dual or multi-flexi" nature concept via "new" goth lines that wouldn't hurt the established game.  I'm curious though.

I'm fine with dual-natured PC races if they're designed that way and it's part of their balance.  Retroactively changing existing races to be dual-nature without a thorough rebalance is the thing I oppose due to the way that affects those races.