The check-in staff of airlines that are members of IATA generally use the IATA Travel Centre to look up the requirements, using an automated process. It is free to use for everyone.
It is up to the check-in staff, probably based on instructions and guidelines issued by the airlines, how strictly to apply these requirements. This is why the traveller in front of me in the queue without a visa may be asked for an onward ticket and I, in the same situation, may not be a asked for it.
P.S. I did not select red for the last paragraph; it just happened. In fact, on the phone I wouldn't be able to change the colour even if I wanted to.