Arelith

Increase the requirements to spot a Planar race- making it a combination of Spot + Lore + d20, and then adding a passive 'bonus' for the ranks of bluff the planar creature has.

E.g. - 'It requires a dc of 60 (d20 + lore + Spot) to detect that a character is plane touched.
For every rank of bluff/perform that character has, the dc is raised by that amount.
So a tiefling with a +10 to bluff would require a score of 70+ to detect.

This means that planar characters can be detected, but only by people who have some business in knowing such things, and if they play their cards right they can probably stay under the radar until they've increased their bluff/perform scores.

This also means the person in question (and here I specificaly mean Planar races) do not have to be -disguised, don't have to fully hide their identity/take on a new identity, if they just wish to hide their race.

if this goes through, if suggest that if the check is failed, then the base race (human, elf etc) is shown instead

Its a nice suggestion, but as I said in a previous post my issue is that, whatever value is required, the information will get out, and spread quickly. With any such mechanic we're inviting players to beat it; challenging them to reach the required values and use it via their RP.

People will make it their business to reveal planetouched races.

For me personally this is more about tieflings than anything else. Part of the fun of playing one is working out how you're going to RP it, hints via RP, descriptions such as always wearing gloves etc. I don't want this taken away, even if it helps overcome the problem of those who simply ignore the fact, besides that is something DMs already have the tools to deal with should they notice it. With adventure XP RPB is a really big deal right now, and its not hard to give a nod to one's heritage via RP.