I've always mixed in enough "not appropriate" challenges so that the default player response is to assume that they might
not be able to handle something, then look for evidence that they can, instead of the other way around. I consciously got the group to there by telling them we were going there, and then racheting up the danger and risk slowly over the course of about six months, until we got it where we wanted it.

(This was with Fantasy Hero years ago instead of D&D. It's easier to do in FH, but the attitude has carried forward ever since.)
Then, since the players really enjoy digging out information, we assume that any fight that they go into without any information could be anything from cakewalk to TPK. Accordingly, the players are quite happy to indulge their preferences and go digging for clues all the time, everywhere. I don't have to slip in a reference to some orcs that got mauled last week by the same monster to show them that it is tough. They go looking for bones, blood, etc. themselves.
Then on those rare occasions where this all falls apart, we'll go roughly one of two ways. If the players are on top of their game, reasonably alert, but have hit something tough without any information, yet decided to try it anyway, then we'll let the chips fall where they may. Sometimes, intuition and heroic action carries the day, and sometimes it doesn't. It helps using a system with some retreat or other desperation options. OTOH, if the players are really tired, and possibly zoning out on me, I'll stop the game and ask them. We spend a couple of minutes recapping what they know, and then they decide. Usually they back away in these circumstances due to the unknown, but they've surprised me, too.
Edit: I should also note that this group runs away from easy encounters (that either seem hard or where they just don't know) about twice as often as they get mauled or near mauled from tough encounters. I let them run away without clueing them in the same way I let them get mauled occasionally. Sometimes, the stuff they get into when running away provides the most memorable part of an evening.
