The primary mechanism for verifying authenticity is the internal product code. From the early 2000s onward, major manufacturers (Nike, Adidas, Puma) integrated specific alphanumeric codes on small internal labels, typically located near the bottom hem or inner collar. Searching this exact code should return images of the specific shirt model, not generic apparel.

* Stitching and Badge Application
  Authentic retro shirts exhibit consistent, continuous stitching patterns. The manufacturer logo and club crest on pre-2010 shirts are typically fully embroidered. Replications frequently display connected threads between separate letters (bridging) or asymmetrical crest placement.

* Sizing Labels and Collar Tags
  Collar tags undergo specific design iterations per season. Replications often apply a generic brand tag spanning multiple decades. The physical location of the size label, the font used, and the presence of the manufacturing country must align with the documented production runs of that specific year.

* Material Density and Sponsor Logos
  The structural fabric of authentic 1990s and 2000s shirts is typically heavier and possesses distinct textural patterns (e.g., jacquard weaving). Sponsor logos from this era are usually raised felt (flock) or distinct, thick PU printing. Replications frequently substitute modern, thin vinyl sublimation, altering the physical weight and texture of the garment.