How can I see if a diamond is genuine?
My honest answer: You can NOT! You need to be an expert and use specialized equipment to make sure a diamond is real. Do not trust ideas like scratching a beer bottle (it may damage the diamond too), thermal conductivity ( conducts equally well), 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.
Tip:If you can not easily find inclusions under a 10 times magnification you should become suspicious, most likely it is not a real diamond. But be aware of the fact that some gas bubbles in cubic zirconium may appear like inclusions if you do not look carefully.
For further information see Diamonds Glossary.