I would write "ç'a été" and I could say "comment ç'a été avec ton examen ?" (How did you do at/get along with your exam) but I would not use it in writing as this particular past time is clunky. (I'd much prefer "comment s'est passé ton examen"). I've never heard a waiter use this phrase but then I've not been to France more than a handful of time. (in Quebec waiters would say "tout était à votre goût ?" or "c'est terminé ?") To answer you question (ça a été /comment c'était - are they the same) the answer is no. If a waiter was actually asking me the first one, I would say "yes" and if he was asking the second I might say "it was delicious" as comment s'était invite long explanation on how you percieved something. If you use them to answer something, rather than ask something (ex C'était merveilleux. ç'a été un désastre) they are the same, meaning "was" but somehow c'était makes me think it happened quickly wereas ça a été was longer. Just my perception, no ide if it has any bearings.