How should a good CX riding position "feel" (alternative to highly scientific measurements...) I'm switching from MTB full-suspension down-hill riding to something more flat - at the moment I'm looking at a CX for 80% road, 20% offroad usage.

Unfortunately, I've never, except for short test rides, used a road bike for any amount of time. So I know somewhat how to sit on it, but I don't have much experience with how it should "feel".

So I'm quite at the mercy of salesmen right now. Sure, I will assume that they have the best intentions, but the problem is that I find it hard to judge whether a specific bike geometry is as it should be. It could well be that there is the perfect bike out there for me, but as I'm not used to the lower riding position I may only notice that after a few weeks of riding, after my body adapts and all such.

Is this the case, in your experience, or should the "correct" position just feel great right off the bat? I am not talking Tour de France performance level here, just standard hobby riding (I am reasonably fit and agile). Are there particular "body signs" I should be looking for, which tell me that the bike is a good fit; in addition to the expert at the store measuring my limbs and all that good stuff?
Most of your riding is on the hoods.  I race CX, city ride CX, and road ride on a CX.  I spend very little time in the drops.  In the city pretty much no time unless into a head wind.

On the hoods out of the saddle it should feel a lot like a mountain bike but no shock.  Come off a curb out of the saddle.  If you feel like you could go out the top it is too small.

Since you are not used to it you should feel only slightly stretched out.