This is a strange question. If someone is sick, they may be out and the theater would not announce it in advance unless it was a long standing illness or something where they knew the actor had to be out for 5-7 days (throat or some other injury that required rest).
Vacations MAY be announced and you might get this info at the box office but not always. If the actor's name is above the title, you can get a refund or try to get an exchange. Theater is live..and therefore, unpredictable when it comes to family emergencies, illnes etc. It is a crap shoot. That is why they have understudies. Sorry, no one can really provide you with an exact answer.
"Life is a lesson in humility"