Shell is the button that will feel cold against your cheek. Horn, less so. Plastic will not feel cold.
Horn can be polished or dull, either way.
On darker colors, it can be much more difficult to tell to an untrained eye. One of the giveaways would be a 'seam' around the button, indicating it is moulded urea. The seam is where the mould comes together. Polyester however is turned from button blanks and will not have that seam.
I'm trying to think of an obvious indicator, but to the untrained eye it may be difficult. If it looks 'too perfect', it's probably not real.
Check the side edge. If it looks a bit 'chattered', it's probably real. If it's a very smooth clean edge, it's likely plastic. This isn't really 100%, but it's a starting point.