As per the https://terraria.wiki.gg/wiki/World#Trivia (emphasis mine):
The leaves of the tree icon when selecting a world will change depending on the status of the world. The left half will always represent the world's naturally spawned evil biome, while the right will be a pure Forest tree. In some secret world seeds, both evil biomes will be native to the world, resulting in a tree that is both Corrupt and Crimson with no pure representation. In Hardmode, the bottom right quarter and half of the left branch will turn yellow, representing The Hallow.
Like you said, there are many other things that change upon defeating the Wall of Flesh and unleashing hardmode in a particular world, as listed on the https://terraria.wiki.gg/wiki/Hardmode page. Personally, I believe the simplest method is to try and destroy Hellstone using Dynamite or Bombs. This is only possible in Hardmode, and the underworld is one of the safest areas in early Hardmode.
Ignoring searching for NPCs which is somewhat random, one can also look for Chlorophyte in the Jungle (spawns even before you break Altars) or fish up some Crates.