The check-in personnel are supposed to know which nationalities are eligible for visa-on-arrival, and thus not ask you for a visa. However, you can print out a copy of this page which shows which nationalities are eligible for visa-on-arrival and keep it with you in case the airline personnel ask you anything.
I once got asked to show my visa at Chennai Airport (India) while checking in for a flight to Australia. I was eligible for visa-on-arrival in Australia, but the check-in personnel apparently didn't know that. (I would guess that most of the people flying that route were either Indians or Australians, and that the person asking me might have been new or inexperienced.) By sheer luck I actually had something to show. As it happened, I had (unnecessarily) applied for an eVisa (which was instantly, automatically granted) online. These eVisas are purely digital. Immigration in Australia scans your passport and the visa information appears in their computer. I had actually printed the receipt email (which is actually not the visa, just a receipt.) I'm not sure if the person at the check-in knew that this was actually not my visa, but she accepted it. It was lucky that I had printed it and taken it with me. (Otherwise, there might have been the hassle of convincing them that I was eligible for visa-on-arrival in Australia.) Similarly, you can avoid some hassle by printing out the page I linked to above.
