Upvote:3
The answer to that is "it depends". I go to Thailand every 6 weeks, give or take, and was never asked for a proof of onward journey. Even when walking/driving to Laos or Cambodia, and coming back shortly after.
But that's because I've established a pattern of short visits (I don't think I ever stayed longer than a week at a time in Thailand) and because I don't look like one of those people who live in Thailand on visa exemptions/tourist visas.
So while one person may be let in without a single question, the next one might be asked for a plane ticket. In your case, if you're going on a tourist visa, you most probably won't be asked for much, or anything at all, really. You've been pre-approved, and that's good enough for Immigration officers. As you may have noticed in your previous visits, they're people of few words.