I think now a days what is worn under the kilt is a matter of personal choice due in part to the influx of women into the military. For example before the pandemic hit Megan Beveridge was the P/M of the Royal Artillery pipe band; she became the first woman in the history of the British army to earn a P/M certificate from the Army School of Piping and Highland Drumming. She also has the distinction of being the lone piper at the Edinburgh Tattoo. There is also a female captain [probably a major now] in the Black Watch, I don't think either of these ladies ask for "proof" when the men fall out kilted. Several years ago a young kilted solider passed out from the heat at I believe a birthday celebration for her majesty the Queen, a gust of wind blew revealing the iconic military boxer shorts. So much for tradition!