Posted By:
HankronI'm assuming you are talking about buying one online as opposed to in person.
Your best bet is to buy from a reputable seller. By this I mean someone who is familiar with the material, has a well defined and acceptable authenticity and return guarantee (if you don't know what it is,ask) and the reputation that he or she will back up the guarantee. Otherwise, you can buy one graded by PSA, SGC or similar top grader.
Most online reprints of the Goudeys either have obvious problems (bad borders for example) or just don't look right. However, as somes are comuter reprints or 'borrowed' images, you can never be too sure what's up. That's why you want to be dealing with reputable sellers when buying a high end and often reprinted item like this.
If you are talking about buying one in person, your best bet is to buy a variety of commons and a blacklight for comparison.
When in doubt, you can post an image or auction link on this page for commment.