You will not. You need to be an expert and use specialised equipment to make sure a diamond is real and genuine. Do not trust ideas like scratching a beer bottle (it may damage the diamond too), thermal conductivity, looking through, comparing weight, etc. If the diamond is accompanied by a certificate, you will know it is real and if it is treated artificially or not.
Round brilliant diamond has the most brilliance, and the most popular. Fancy diamonds look more elegant in larger sizes. Also, they tend to look larger than they are by virtue of their shape. The choice of shape is also governed by the shape of the hand and Diamond Colour and Diamond Clarity. Round brilliant diamond hides defects and yellow tints the best.
The four C's are Carat (weight), Colour, Clarity and Cut of the stone. Check this page for more details.
The most desirable colour of the diamond is white. The Colour scale ranges from D to Z, where D is the whitest of the white and hence most expensive. However, colours between E to H are regarded as very white, and you cannot make out the difference in colour once a diamond is set. However, the diamond colour becomes more obvious as in larger size diamonds, or in shapes other than the round brilliant. For instance, the Asscher and the emerald cuts are more see-through with large facets and require a higher colour and clarity than other shapes. Colour is something that can be discerned with the naked eye with practice (such as looking at the engagement ring every day. Hence, it is advisable to buy as white a diamond as you can afford in your carat range, in an eye clean or better clarity. If a diamond has slight blue fluorescence, it could render your H or J colour whiter. Also, if the stone is very well cut, it makes both the clarity and colour look better. Clarity is not a factor you can judge only by looking at the diamond under magnification. Step cut diamond shapes like emerald and Asscher cuts require a higher quality of colour and clarity as they have large see-through facets which make it easy to see any imperfections.