Was asked at check in desk by British Airways staff if I was travelling on a visa, then they had a good look at it. Wasn't asked for onward or return travel details. UK passport. 2 month single entry tourist visa. (no one ever asked me this before when leaving the UK) This happened 20 mins ago.
Get a 2 month one before you arrive. Yes, you can extend for 60 days, but it's 500 baht in taxis too (assuming you're in Bangkok - it is not accessible by train etc) plus could have to do some waiting around once there. Plus the traffic. Not worth the hassle....Get a visa before you arrive, I'd say. Even if it was more expensive, anything is better than waiting around and sitting in traffic, in my opinion.