Copper Nickel Plancet

You can ask. Get it graded. Then everyone will know its authentic. ebay or any auction site will sell good. I'm not sure on what its worth.It depends on the grade of the coin.

Variables: Weight, color, how it fits on the planchet, appearance, alignment. Images would be the best aid to help now.

And it will cost a significant percentage of the value of the coin (if real).

Here is an example of a real Nickel on a Cent planchet.

Note how the design goes over an edge and some letters are missing? The is because the planchet is smaller that the nickel die. The strike can be more on center and still leave edges of devices missing as it goes over the edge of the planchet.
It a coin were Squeezed, It would have a reversed/mirrored image instead of a normal strike.

Note the reversed images over the other coins devices. It they used an un-struck planchet, the normal devices may be missing, but a reversed design would indicate a garage job.
Weight would be something I would check. Does it weight like a normal cent or like a nickel? Thickness could tell if it is the same thickness of a cent or nickel?
Color: Is the coin a copper color like a cent or a dry looking brown? Some nickels left out side tarnish with a color like red primer. Totally flat and lacking luster. So here are few other thing to compare your coin with.