One of the first things that seem to confirm the diffusionists view are pyramids. This is in the first place because of the straight forward design, lacking any ornaments. It is possible, that the pyramid had been build over an much earlier sacred site.
Sceptics argue that the American pyramids are very much unlike the ones in Egypt.
In Egypt it is the other way around. Additionaly, the timeframe doesn't fit. But in Egypt, pyramidbuilding included step-pyramids and continued up to the 17th dynasty and stopped around 1550 BC, and pyramidbuilding in Mesoamerica began much earlier, with the Olmecs. Thanks to the oil industry, nothing of that survived. Additionally, even older pyramids were found in Southamerica, build from adobe bricks by people like the Tucume and the Huaca.