Some people after handling enough karat gold can determine between gold plated items and look alike metals, but it is difficult to tell the karat. You can see this if you sell enough too, most buyer's don't test every item, only selected, suspected, or unmarked items.
The test is only needed to trade it for money. Some people have the ability to quickly spot any sign that the item is not gold, there's a feel to gold, you can see that it has luster and a closer look will reveal the crystalline structure that gold plating doesn't have, some people secrete enough acid in their skin they rub the gold to see it it transfers any black color to their skin, we know what lacquer looks like and how it is applied.
As for making a determinenation from a photo, we go by what the poster writes, the photos, and our questions. It takes a lot to learn the characteristics of gold.