You can't tell if he's bluffing unless you take action that forces him to reveal his hand, at which point it could be too late and the nukes are already in the sky.
Enforcing the no fly zone means shooting down Russian planes which would certainly escalate things.
By the time we find out that Ukrain is just a stepping stone, we may realize we should have stopped him sooner.
We know now that Crimea was not enough for instance. That is information that should be useful to us, yet we seem to ignore the pattern forming right in front of us.
He he's not bluffing with nukes, then we shouldn't expect him to stop here and need to act sooner rather than later.