No, Thai immigration won't ask.

ANA may ask to see proof that you are leaving within the 30 day visa exempt period prior to your outbound flight.

Awesome! Thank you! Yeah, hadn't thought about ANA asking. :)

From my experience they can ask and I have been asked for proof of onward journey. In my case I arrived on a flight with no return booked, as I was going overland to Cambodia.

For low risk travellers from low risk countries ​​​​​they probably don't ask additional questions commonly, and it may only require a verbal confirmation rather than documents. The first line of defence may be the airline undertaking basic checks. But you should always be prepared at immigration to discuss or evidence of your plans to visit a country, how you will fund yourself, and how and when you will leave. Whether immigration will ask is another question but being prepared is always sensible. A backpacker with multiple visits to Thailand might have more chance of being asked more questions than an apparent business traveller. Much of what an immigration officer is trained to do is to assess risk and to look out for signs of lying and people being economical with the truth.

Cool, thank you! Yeah, I can provide proof (well, I can show them screens on my phone with my reservation in the United app!). I'm an older gent, and tend to dress nicely... so pretty sure I won't get mistaken for a young hip backpacking type! :) LOL!

the probability that ANA check in agent ask will be much higher than that the immigration officer will ask

usually check in agents for flights to Thailand know well the rule, that for passengers with visa exemption a return or onward flight ticket is required. And I have found the agents in Japan and Korea more serious about such rules than anywhere else...
immigration usually does not ask unless you have many recent entry stamps to Thailand and/or look like a backpacker

Got it, thank you! I appreciate it! I'll make sure to have it handy in case I'm asked.

BTW - is there something that gets flagged to them in their system so they know to ask? Or can they just see that a given passenger is flying on a 1-way ticket and they can just be proactive about it?

I have been in and out of Thailand almost mote that twice a month and they have never asked me anything apart from the finger prints and photo. And of course the boarding card on your arrival flight.
Yes the airlines ask for confirmation of flight out of BKK at check in.

Maybe some combination of both, depending on the airline. Info like that is in TIMATIC.

-David

Ah, got it! Thank you! :)

well, I guess when they check in a passenger, they see, that there is no return flight on the ticket....that triggers the question, whether there is a visa of some kind or an other onward flight...I had to help them find the visa (or in my case EOS) in my passport numerous times when I checked in on flights bound to BKK

I get that often as well, though strangely not on the past couple of trips - but cannot blame them for not being able to find the right one as my passport has a multitude of different Thai stamps for visa transfer, extensions, re-entry permits, a cancellation and a conversion :o

If I were you I’d be prepared with the TG ticket number. If ANA asks you, they may or may not be satisfied with other evidence (such as the reservation viewed on your phone, which typically doesn’t include ticket numbers).

I'll be traveling through BKK in July with a 7.45HR transit. I will be in J. Tossing up weather i should book the Miracle Transit hotel which I read is appalling in nearly every aspect or The Novotel Airport hotel which is probably nicer but run down. Both seem outrageously priced. Will it be a hassle getting a transit visa, even with FT access(Aussie passport) OR should I just lounge it out, get a massage etc? I don't think any of the oneworld lounges have rest or sleeping areas or the Miracle lounges which Il'l also have access to-unless someone here knows better?

If you have thru checked baggage why don't you take the train to downtown BKK and then get a good massage? You do not need a visa to land in BKK.

Assuming the transit is during day-time (and checked in luggage as mentioned), I would strongly suggest this as well - go into town, get a massage and a nice meal before heading back.

Fast track immigration inbound usually has very little wait, whereas outbound could be more variable (nothing to worry about though).