"Experience. If I gave you a hundred similar flavors, only one of which you'd tasted before, you wouldn't always get it right. However, if I did the same test with one flavor in common over and over again, your chance of getting it right would increase. With no additional sensory input, an individual can be trained to detect their own home world almost 100% of the time. Even without training, frequent travel raises the percentages. The empty worlds are more significant than variations on ones with life though."