@Susie ********
in the US, the consulate always asks for proof of onward travel at the time of tourist visa application now. I don't know about other countries.
The rules for Thailand say you must be able to show proof of onward travel within the limits of your entry time period, and it's the airline's responsibility to ensure you meet the requirements before letting you board.
Whether they do or not is random depends on the specific agents you deal with, how busy it is, and a host of other factors.
Since it's the rule, isn't it best to be prepared to show documentation to meet the rules?