I am more of a world simulationist DM than a story-based DM where the world fits itself around the party. There are all kinds of things above (and below!) the pcs' levels in my game, and pcs run into them fairly often. I run a hardcore sandbox, and there are usually some clues to how dangerous a given area is. For instance, random encounters might be with signs of monsters (e.g. dragon scales, catoblepas droppings, etc) rather than monsters themselves.
Ultimately, evaluating the danger level of a given encounter is something that rests pretty thoroughly on the pcs. Things like the number of enemies, how big and nasty they look, the kind of armor they have or weapons they're carrying, their posture (alert and watchful vs. half-asleep), etc., all serve to offer some clues. Players who attack everything usually learn fairly quickly that they can't always get away with that, because they end up getting their asses kicked or even getting their characters killed.
For one concrete example, a pc ranger in my game kept wandering off on his own during a spring in which there were lots of griffons in the sky. Eventually, he got attacked by three of them and eaten.