I entered Thailand over a dozen times last year, BKK & CNX, and haven't been asked about onwards flights. Sometimes I already had one booked, other times I didn't but could easily book one on the spot if directed to by the IO.
I was asked on 17 Dec if I had the required
*****
baht, and as I pulled my phone out to show the IO she just handed me my passport back and ushered me on.