Many supposed missing clad layers are electro-plating experiments and it’s not difficult to accomplish; as can be seen in the video. One red flag against a coin having a clad layer is when both sides of the coin appear to be missing the clad layer. It is possible but extremely rare for both clad layers to be missing.
Also sintered or improperly annealed planchet can also be mistook for a missing clad layer but these will weigh normal and be a normal thickness.
Also look at the coin edges for signs of prying the clad layer off, they might be small marks but they will be there. A clad coin has a copper core sandwiched between two nickel/copper layers and examination of the edge is important.
*Nickels, cents and silver coins are made of a solid alloy so don’t have a clad layer.
Related Link – Missing Clad Layer Mint Error