How does one determine whether a particular Thai person is, or has been declared, legally bankrupt ?

First stop would be the national Credit Bureau https://www.ncb.co.th/Default.htm they have offices all over.

I know one can get one's own credit report for a small fee but I'm not sure if anyone other than a financial institution can obtain another person's record. Time to ask the NCB themselves I suggest.

Thanks Crossy . . that's very helpful . . I hadn't managed to get that lead anywhere else.