Can someone tell me what connection types are most commonly found on the back of a Thai TV? S-video, SCART, RCA or something else? I want to be able to connect a portable DVD player on my next visit and want to make sure I have the correct cable.

Yellow RCA Cable - every Thai TV will have this. Some have S-Video and Some may have Component.

Yellow RCA Cable - every Thai TV will have this. Some have S-Video and Some may have Component.

As above ^^^

Unless the telly is really old, then you'll be lumbered with antenna only, RCA modulators (so you can plug into the antenna connection) are available but uncommon.

Pretty well none have SCART (or it's euro name Peritel).

Unless your DVD player has weird connectors any adapter you can imagine is available here

EDIT Do note that hotels often disable access to the AV inputs of the TV in which case you're buggered whatever the connector