The reason you are asked at check in or boarding is that if you arrive in a country without the correct valid passport and visa, or proof of onward journey, the airline will need to arrange for you to be returned to your point of departure.

So a last minute seat from BKK to IAD pretty expensive. Also the airlines can be fined up to (i think) $10000 USD for carrying you to a destination without the correct documents

So the airlines do police this pretty smartly.

So the general rule is if you are travelling on a one way ticket - have the return or onward available for sighting.

As wifi or phone coverage is not always working or available, I alway print out my tickets as you may be required to show them at check in, boarding or a immigration or security check point along the way

Travel is wonderful its a learning experience.