Just buy them from a reputable store and not worry about them being fake. 
If you have a charger that has a test mode, you could check to see about how many mAh they have. Anything significantly lower would indicate a fake.
You can also compare them to other tests people have posted. Here’s a test on the Sanyo NCR18650GA: Test/Review of Sanyo/Panasonic NCR18650GA 3500mAh (Red)