Assuming you used the techniques from How can DMs effectively telegraph specific dangers in D&D? and How can I make my PCs flee? to telegraph danger and the need to improve the characters skills/level before they can face the encounter.
How do you show players, without meta-gaming, that they are now ready to face said danger?
There are some examples where the players themselves recognize that they can now circumvent the difficulty of the encounter. For example (in D&D 5e), a Cleric that just learned Greater Restoration, which can cure petrification, will probably get the idea that the Medusa fight is not that dangerous anymore.
Other examples like an Adult Dragon that earlier munched on an adventuring party with magic gear do not have such "triggers". The Dragon is still way bigger than them and the players did not know the exact level of the adventuring party.