True indigo yarn is dyed with natural or synthetic indigo dyes, but can also be dyed with sulfur dyes, vat dyes, or direct dyes. The methods for identifying genuine imitation indigo yarn include:

① Lightly apply a drop of concentrated nitric acid solution onto the dyed sample. If it is indigo yarn, white spots will immediately appear with green edges.

② After igniting the dyed sample on the flame, immediately leave the flame and place it in a white porcelain plate. Due to the heat sublimation of indigo, blue marks are left on the white porcelain plate, while other dyes do not have this phenomenon.

③ Cut the dyed sample into pieces and place it in a glass test tube, then heat dry it on an alcohol lamp. Indigo is heated and emits red gas. After cooling in a glass test tube, there is a blue precipitate.

④ Stained samples can be treated by circulating dimethylformamide (DMF) solution. If it is genuine indigo yarn, the dye on the sample can be completely extracted and appear colorless, while the dye on other imitations cannot be completely extracted.