• The price tag. The fake ones will be much more cheaper than the originals with a price which is reduced with more than $100 dollars!
• The booting. The fake Galaxy S8 phones boot slower than the originals.
• The fake ones are much more heavier than the originals. They can actually be 2 times heavier.
• It will be good to check the Android version the phone is running under. The fake ones can be running with Android 6.0 while the originals can be running under Android 7.0 with a non removable battery of 3000mAh.
• Galaxy S8, S8 plus original display is OLED of which is high quality. The fake ones display is LCD and their quality is poor, with a black border at the bottom and top of the screen, of which is not included in the originals.
• It will be very wise to check on the placement of the Flash, Fingerprint sensor and the Camera. If they are not placed like those in seen in Samsung official pictures and advertisements then that's a clear indication of a fake product. The Camera quality and resolution also tends to be poor. As to the Fingerprint sensor, they are placed on the right of the Camera. The originals primary Camera is 12-megapixels and 8-megapixels in front for sefies.
• In addition to the above information about the Fingerprint sensors, they may also not work, and if they do, they can unlock the phone with anyone's fingers. The other thing which may not function properly is the Iris sensor.
• The originals have a new Bixby button right under the Volume buttons. Their function is to activate the new assistant. The fake ones may lack them or have them, but not function properly.
• Original Galaxy S8, S8 plus have a USB C cable, Documentation, SIM tool, ear buds which are new, a 2x OTG Connectors and they come in an all black box. The label on the front is printed S8 only. The fake ones may lack all these products inside.
• The logo on a fake Samsung Galaxy S8, S8 plus seem to be raised slightly, one may confuse it with a sticker. Their screens also are more reflective and appear lighter than the originals.
The original Galaxy S8 have a flash, are GPS enabled, they use a 3.5mm Headphones, they have a slot for only one SIM card, they are 3G and 4G/LTE enabled, and have sensors such as Proximity sensors, Ambient light sensor, Gyroscope, Accelerometer and Compass Magnetometer. The original Galaxy phone also has a powerful 1.9GHz Octa-Core Exynos 8859 processor with a RAM of 4GB. The internal storage is 64GB which is expandable up to 256GB through a MicroSD card.
Like they say, cheap is expensive. It's better to get a real deal phone which comes also with a warranty.