Studs and clutch power
Genuine LEGO studs have a consistent diameter and a slightly polished top. Clones often feel waxy, show seam lines on studs, or clutch too tightly/loosely on official bricks.
Snap a suspect plate onto a known-good LEGO brick. If it feels unusually loose or requires force that bends the part, set it aside for closer inspection.
Color and transparency
LEGO color batches are stable within an era; clones sometimes look oversaturated or slightly milky in transparent elements. Compare like with like — old gray differs from bluish gray on purpose.
Catalog photos help: if your green 1×2 plate looks neon compared to reference images on the part page, verify the mold number underneath.
Mold numbers and branding
Most LEGO elements carry a tiny molded design ID. Missing numbers, blurred digits, or alternate font styles are red flags — especially on plates and tiles where clones are common.
Minifigures are frequently cloned. Check print alignment on torsos, the LEGO stamp inside legs, and whether accessories match official molds.
Why accurate identification matters
Mixing clones into "100% LEGO" listings creates disputes. Sort suspects into a separate pile, label listings honestly, and use catalog matches to show which pieces are verified official molds.
Tools like BrickID confirm whether a shape exists in the official catalog — they do not prove brand by themselves, but they give you a reference shape and parent-set list to compare against.