I see that on Amazon you can find a lot of minifigures but I can't tell if they are cheap knock-offs or not.
If you can't hold them in your hand and inspect them closely for the usual LEGO production marks, as it would be the case with buying from Amazon, your best bet would be asking the seller and checking whether the individual parts exist as official LEGO pieces. A custom piece that doesn't appear in any official sets or catalogues clearly indicates a third-party make. Note however that the inverse is not true, if all parts of the figure can be found in official sets, they still could be custom-made. At this point it all comes down to the honesty (well-informedness?) of the seller and your trust in her/him.