As metal prices go up fakes get more and more advanced. If the coin was filled it would have the exact same luster and not be worth nearly as much. Even 1oz silver rounds are starting to be faked now.
I agree about outer apearances. But no fake gold coin will pass a set of calipers and scales. In theory tungsten could be used as a filler but so far there have been zero credible reports of anything as small as a 1oz coin being filled with tungsten mostly due to how hard it is to work the stuff.
With large bars, like 100oz and up tungsten is a concern. People are using ultrasound to see if the bars are consistent throughout.
Silver 1oz have been faked and faked very well for a long time. Unlike gold they can get the density just right by the right mix of base metals.
The Fisch kits are pretty good for detecting fake coins.