1 - When it comes to gold, ALWAYS buy from a trusted seller. Its just not worth the 10$ you would save if were to receive a fake 900$ coin from a relatively shaddy seller.. Ive been lurking on eBay for several years now and I have never seen a fake or fraudulent account with more than 10 positive feedback, (usualy from accounts with random characters, so obviously fake themselves) . Stick to purchasing your gold from people with over 50 feedback and is 100% positive. This GREATLY reduces your chances of getting a fake gold coin or bar.
2 - Now lets say you just received your coin or bar or nugget. Weight it. Buy a digital scale if you need to and weight the coin, it will be worth it in the long run, these types of scales have multiple uses. (As a reference -  ounce of gold, silver, platinum, palladium, etc.. weights 31.1 grams.). After weighting the item, measure it. Google the particular item along with the "dimensions" and measure you item to make sure its EXACTLY the same size. Gold is unique in the way that it is very heavy for its size. A gold coin in noticeably heavy when its laying in the palm of your hand, something that is impossible to immitate if the size is exactly the same.
3 - As a last resort, if you are still anxious about your purchase, you can bring it to a coin shop or a trusted jeweler and have the owner appraise its value.