The stuff you're looking at in the picture above is pyrite – aka, "fool's gold." Apparently we have a lot of it around us, because we find it quite often. It even fooled us a little the first time.
We've found the easiest way to tell it apart from real gold is by testing how brittle it is. Gold is not brittle at all; it's soft and malleable. If you hit a round nugget of gold with a hammer, it won't break; rather, it will get flatter and flatter the more you hit it, until it eventually becomes a disc.
Pyrite, on the other hand, is extremely hard and brittle. It often looks like little specs of gold or gold dust trapped in another rock. If you hit pyrite with a hammer, it shatters. It's so brittle, you can actually break it with your teeth, or sometimes even your fingers; hence old-timers biting down on gold to check the authenticity.
Sometimes, you can also tell fool's gold from real gold simply by looking at it. Real gold always looks golden under any lighting, but pyrite only looks like gold from the right angle in the right lighting.