Look, I'm not gonna tell you the DC so you can just make a note and come back later knowing you can pass. You're gonna have to see what happens and figure it out. The only time I will ever tell a player they can't do something, is if they are trying to do something that is against the rules of the game. I might inform them of potential consequences of their actions if they don't seem to quite understand something that should be obvious, but that's about it. I also use critical fail and success with skill checks in my games, to make things more interesting. So even if the DC is impossible, a NAT20 may still get you through.