You don't find your company's DNA in a mission statement. You find it in the stories people tell, the moments they're proud of, the way decisions are made.
Start with conversations: three to five staff members from different departments, plus leadership. Don't ask 'what are your core values?' but 'when were you most proud of this company?' and 'what would you never do?'. Also look at rituals, language, humour.
We do this as standard on every concept project — a two-hour DNA session delivers more than ten pages of strategy.