There are three giveaways, 2 under the tank and 1 inside the fuel filler hole.
1. The 4 indents for the carb caps in a factory tank all line up in a row The Yamiya tank has the 2 inner indents offset towards the front and thay are big indents
2. The petcock pad is noticeably raised and the raised part is symmetrical whereas the factory tank is flatter and isn't symmetrical. Even the double wrinkle raised petcock pad isn't as symmetrical. The Yamiya pad stands out like dogs balls
3 There are 2 features of a factory wrinkle tank. The wrinkles themselves and the dimple are easily noticed when one looks through the filler hole. The dimple on a Yamiya looks to be formed by a pin punch. In the centre of the dimple you can see a sharper pin type depression. You can also notice this on the underside as well. Factory tank has more of a rounder depressed dimple.
The main giveaway are the 4 carb indent alignments, but I've included the other clear differences for completeness. There are a few more subtle differences but I really can't recall what they are. The tanks are a good alternative as they are relatively cheap and are new (un-dented metal). I think the one being sold is just way overpriced.