They are fairly easy to pick out simply based on the way money flows in and out of the addresses. Also, they become known over time as customers share them the

They are fairly easy to pick out simply based on the way money flows in and out of the addresses. Also, they become known over time as customers share them then they are eventually labeled by companies (exchanges included) performing blockchain analytics. Many of the public blockchain explorers show labels for popular addresses. I've seen labels on big exchanges, Satoshi's coins, seized silk road coins, coins from hacks, etc.