"My main point is that using physical clues such as skin color to determine a person's race can get real complicated and does create difficulties for those people in part because of internal and external racial assumptions and racial prejudice."

Until relatively recently, if you wanted to assess the ethnicity of an audience at an arts event, it was considered appropriate to do a "head count", identifying all of the non-white attenders and possibly trying to categorise them in a very broad way. This was considered necessary because there were usually not that many of them, to be honest, and doing surveys, where the audience can choose (or not) to take part, might not reveal the true proportions of ethnically diverse attenders.

However, nowadays this is no longer considered acceptable, because there are too many assumptions that can be made by the person doing the counting, and because ethnicity is so complex - so the only credible method at the moment is self-identification.