When and how to announce difficulty levels to players is a common question on forums, as it's one of those things left open in the core books.

In short, the book says you can tell them, but you don’t have to and that it’s good telling DC to starting players.

Here is how I do it.

Starting Out
When you start a game, whether it’s with new players or old ones, you should disclose task difficulty. It will allow players to get a feel for the world and internalize how their resource spending will look. What is hard, what is easy, when they need to spend effort. It is important that players grok that from the start.

Keep that up for the whole first session and once the finale comes, stop. This will have a nice secondary effect where your finale will have more tension.

Here is the first catch that you need to look out for. The Scan ability. If you have a player with Scan, you don’t want to invalidate his ability by straight up telling DC numbers.

There are two ways around this in the first session. You either remind the player to always spend his action to activate Scan or treat it like an Enabler for the first session (sans finale), sparing his actions. You will need to communicate this decision to the player beforehand though, in order to avoid confusion later on.

So now that your players have a good sense of how things work in your world, we come about the second catch.

Character Expertise
Characters will have a sense of challenge for some tasks even if players don’t. Here is an example: If you were to stand next to a tall wall (in real life), you would have a good sense of how difficult it would be to climb it. If you were a trained climber, you would know exactly.

This means that you need to think if any of the characters can determine the task difficulty “passively”, on their own, taking into account their background, story progression, and any skills. This is something you want to train your players to ask as well. “Can my character discern this difficulty, because he is such and such, did this and has that?”

Because you will forget.

More General Levels
Soon enough you will get into situations where characters or players have a more general, not that precise idea how difficult a task is. In these situations, I split the range in two or three and say something like: “It looks challenging, but it might be easier” for 1-5 or: “It looks challenging, but it might be worse then it looks, even impossible” for 5-10. Same approach if you split the difficulty range into three parts. It’s worth noting here that although I give an in-world description of the perceived difficulty, players know what it’s about on the meta level, so as soon as I say: “Looks demanding, but might be not”, they know, without a doubt, that I mean DC 3 or lower.

Impossible Difficulties
As stated in the rules DC 7-10 are not rolled, unless a player can bring it into d20 range. In practice, it means you must communicate this fact to players at one point. You don’t want to wait for the player to spend all she has, roll a natural 20 and only then shrug and tell her, sorry not enough. That’s a fun breaker right there, don’t do that :)

Every time you have a DC 7+, after the player states all her Easings, but before she rolls, if the target number is not in a d20 range, you stop and say: “Unfortunately that will not be enough for success.”

A Bit of Psychology for the End
When the DC is unknown players will tend to overspend their pool points, due to uncertainty. You can use this to great effect when presenting new monsters or weird prior world magic in order to keep suspense in the air.

Sometimes even Scan will not pick up crazy prior world stuff.