With the latest update to Commander rules I'm a little confused by this:
Your commander is always your commander regardless of where it is or its status. That means enough hits from a face-down commander can kill you.

Example:
My opponent has 19 damage from my comander but he is at healthy 100 life. Now I attack with my manifest and he takes 2. Than I suddenly turn it over with GOTCHA! you've been dealt 21 commander damage. Is that correct how it works now or I'm supposed to inform my opponent as soon as I manifest my guy that it is actually my commander because it is a public information?

I know it seems like very corner case situation with regard of the other changes but I haven't found better forum to ask my question. Thanks.

Your commander effectively has a glowing halo around it that says “Hey, look at me! I'm a commander!”, even when face-down. As such, if you manifest your commander, your opponent will know that you have done so.

Depending on who your commander is, when viewed by your opponents this halo may take the form of an alternating series of red and white concentric circles.