目印 and 標識: a landmark and an information sign
Compare a red roof used as a landmark with a road sign checked before a turn, distinguishing a location clue from displayed information.
目印 serves as a clue to a location
目印 (めじるし) is something used as a clue to identify a location. “赤い屋根を目印にして道を曲がった。” says that the speaker used the red roof as a landmark when turning off the road.
This does not mean that instructions were written on the roof. The red roof’s role was to provide a recognizable clue for finding the way.
標識 displays information
標識 (ひょうしき) is a sign displaying information such as guidance or rules. “道路の標識を確認して右へ曲がった。” says that the speaker checked the road sign and turned right.
The road sign is what was checked, and turning right is the action that followed. The sentence gives no specific wording or symbol on the sign, so do not invent a traffic rule for it.
Look at the role rather than whether it is artificial
目印 and 標識 are not mutually exclusive categories of natural and artificial objects. A sign can also be used as a landmark, so their roles can overlap.
In these examples, distinguish using the roof as a clue from checking information displayed on a sign. Read the location-clue role and the information role without adding actual rules for interpreting road signs.
Does using the red roof as a 目印 mean that instructions were written on it?
Show the answer and why
No. It means that the roof was used as a clue to identify a location. A 標識 displays information such as guidance, but that same sign can also serve as a 目印 for finding a place.