It's doubtful that if this were to gain that much traction (which it honestly doesn't look like it will) that the secret could be kept for particularly long anyway.

I'm not really sure what would make a good passive sign to indicate Effective Altruism. One assumes that things like the way we talk and show cooperative rational attitudes might be a reasonable giveaway for the more observant.

We could borrow the idea of colours, and wear something that is conspicuously, say, silver, because silver is representative of knights in shining armour or something like that, but I don't know if this wouldn't turn into a fad or trend rather than a serious signal.