Maybe some cuts/fits are a dead giveaway, but I'd honestly like to see someone point out 2% Lycra in combed cotton denim with 100% accuracy. I don't think it can be done. As I understand it, FTC regulations require any fiber content OVER 2% to be listed (i.e., apparel with cashmere content under 2% cannot be called cashmere fabric). Lycra is the one exception, because its performance abililties at just 2% are significant enough to warrant its inclusion in hang tags and labels. But stretch jeans looking weird on guys? Depends on the style, I'd say; not on the lycra itself.