Yes it is not, plenty of soft cues are signals - today's generation isn't good at identifying them ( a good thing!).

Meat/no-meat is the most common signal, certainly not the only one, surname would be another, language (ethnolect or dialect) is sometimes another.

There are no perfect methods short of asking, some older people and more overtly racists (even they aren't aware) straight up do ask.

Also most racists only really care whether you are their caste or not, it doesn't matter what you actually are, so classification into their group or not is easier than do it precisely.