It would be great if someone could authenticate this cheap China-made models that are flooding the local market. For the guy one the street, it is not easy for him to tell whether a phone is a genuine Samsung Galaxy S3/S4 or not. On the surface, I have been fooled too but we need to establish how do you spot (and avoid) a fake Android phone.
Let me give a shot at what to look for when checking if a phone is a fake before buying:
1. It is way cheaper than the standard market price. Get to look around and check what the price for that type of a device is and hence roughly how much you should be paying for it.
2. The specs and dimensions of the device being looked at are different from the official figures. This will require you to physically check the device (avoid doing high value purchases online where you can't inspect the product)
3. Go to the settings panel and check the User Information that it matches with the claimed specs by the manufacturer.
4. In the case of Samsung, look for the Samsung Apps app as only genuine/original phones from this brand will carry this app. Most replicas will have the name "Apps" only with no Samsung.
Hope that helps you in making a decision as to whether to buy an Android phone or not.