Feel the fabric of the jersey carefully. An authentic Mitchell & Ness jersey will feel heavy in your hands, and the material will be very sturdy. Knock-offs tend to be quite lightweight.
Examine the team's name--not just the player's name--to ensure it's a professional sports team. Mitchell & Ness does not make any college or high school jerseys. They don't even make older jerseys of current professionals who were once amateur players.
Look for details at the bottom left portion of the jersey. Verify that a label is embroidered onto the jersey that reads "Nostagia Co. Mitchell & Ness, Philadelphia, PA" alongside a small, embroidered label with the player name and the year the jersey style was worn.
Inspect the front and back of the jersey. Make sure it does not have any embroidery commemorating a championship or any milestone These types of jerseys are typically celebratory styles presented to players after a championship game or for novelty purposes for fans; however, they are not authentic styles. Mitchell & Ness does not make them.
Check the size tag. Make sure that it's a numbered size (typically between 40 and 64). If the jersey has a lettered size, it's definitely a counterfeit because authentic Mitchell & Ness jerseys are more precisely sized.