3hole...
By how the aircraft behaves, of course!
It's my sincerest hope that a professional "pilot" would never have to ask that question! I'm presuming those of you asking it are not pilots, because I can't imagine a pilot who could ever not know if he's stalled, PDQ, after the fact. The fact that recent years have shown us that those kind of pilots do exist - even while they earn their Darwin awards, ought to cause us all to have a really good look.... If any pilots reading here have any doubts, have no doubt that you DO need some training.
We do not all have your "Chuck Yeager" skill level. The rest of us non-aces require extensive training.
This was a stall in an a/c with degraded instruments, ALT LAW, no visual in chop. Not a trivial case. The inexperienced PF did try stick forward however a/c still stays nose up no response, as elevator is at -30 deg due to full THS! So behaviour is bizarre and only explained if you have a full understanding of the situation in 60 seconds. Even then you would be faced with stall recovery - also not trained for and not trivial.
Bus pilots have stated that ALT LAW behaves like no other a/c you have flown! So if you have not tried it don't you think it is a bit presumptive/over simplistic to "see how a/c behaves".
Even the QF72 CAP with the help of experience, day time, normal law, good weather still doubted varacity of all his instruments. Fortunately he could correctly ignore his stall warnings. IMHO given the details of AF447 incident it was a difficult and exceptional situation.