I was recently informed that kilts only came into wide spread use in and around the end of the 16th century. How did clans identfy each other before kilts? Were their just flags? Or was it just like everyone knew everyone?
Whilst it's unknown whether or not the kilt was invented outright in the 16th century or formed out of earlier garments, the question is moot because the concept of Clan-specific tartans was invented by RR McIan for his book The Clans of the Scottish Highlands, 1845. By the time these were invented, there was no real need to identify clans.
When identifying a warrior, Scottish clans would use emblems as every other faction in Europe would to distinguish themselves, primarily arms and banners.
Source: Reconstructing History - The Evolution of the Kilt, What the Highlanders Never Wore
Source: legislation.gov.uk - Lord Lyon King of Arms Act 1592