There is a spectrum of replica quality in the watch world. Some watches are very obvious to spot while others would be impossible to spot from a photo. There is also the concept of a frankenwatch: a watch that is a blend of original and non-original parts. Assuming the builder has done their research and due diligence with part selection, you would likely not be able to identify a frankenwatch from an original if they were sitting right in front of you. As a result, all watch exchange forums that I have visited have suggest that authenticity of a purchased watch be verified from a authorized dealer for that brand.

To expand on this, serious watch collectors will keep service records from authorized dealers as proof of authenticity. For example, many models of Rolex have “service” parts that differ from the parts originally used in the model. Pricing general follows the hierarchy: original parts > oem service parts > non-oem replacement parts. The pricing gap between each of those steps can be in the tens of thousands. So if eBay authenticates a Rolex as original and you later on find out it has some non-oem replacement parts, you’ve likely lost a significant amount of money on that transaction.

It's still possible!

It'd help us to open the watch, naturally, but we can still rely on pics. Make no mistake though — the watches can be 99.5% perfect, but (at least for the watches we're handling, which have high values) the compromise in manufacturing processes is spotted one way or another

It's worth pointing out that the discussion about replacing pieces of a fake watch with authentic pieces (making a so-called 'franken' watch, as rep watch connoisseurs call it) is a totally separate discussion