Phil Round has just published this article discussing the identification criteria of Northern Boobook, a species split by Ben King in 2002. Phil mist-netted one in March this year at Laem Pak Bia, the first confirmed for Thailand, and as a result of research off the back of that record, as described in the article, it seems that Brown Boobook (the common resident species in Thailand) can be separated from Northern Boobook in the field - it was previously though to be separable only on biometrics (most notably, wing length) and call. The key in-the-field feature is the shape of the markings on the bird's lower breast and belly: Brown Boobook shows obvious heart-shaped spots, whilst Northern shows tear-drop shaped spots.
Photos illustrating Phil's article clearly demonstrate this difference, and a look at the Oriental Bird Images database show that this feature holds true (note that in the database of Brown Bookbook photos on OBI, picture number 5 is a mis-labelled Northern, photographed in Hong Kong in April 2011).
Confirmed/suspected records of Northern Boobook in Thailand now seem to consist of two birds photographed around Bangkok (one in Oct 2008 and one in Feb 2011) and the bird that Phil trapped in March 2011.
I saw this "Brown Boobook" at Suan Rot Fai in April 2010 (see my post here), and on telling Phil of the record (quite a good local record and the only one I have seen on the patch) he mentioned the possibility of it being Northern, given the fact that it was probably a migrant, however without the bird's biometrics it was pure speculation (the underpart pattern ID feature was unknown at that stage). However, upon reviewing the images of the bird (see below) in light of Phil's article it seems clear that this bird has only tear-drop shaped spotting and so I think it is a Northern Boobook, which based on current records would make it the 2nd record for Thailand (after the Oct 2008 bird).
Of course, given the fact that this ID feature has only just come to light it is very likely that other records of Northern Boobook will emerge, identified from photographs, so it remains to be seen what the species' true status is in Thailand. Round (2008) states that Northern Boobook breeds in China and winters in the Philippines and Indonesia, and speculated that it may occur in Thailand.
EDIT, 2nd Oct: Phil Round has commented that "assuming plumage features are reliable (and I'm pretty sure they are), I would say this is 100% a Northern Boobook." He goes on to say "The first Thai record is presumably a bird listed in Deignan's (1963) check-list as Ninox scutulata florensis (either a synonym of japonica or a subspecies of japonica) from Trang Province. I think there is another from Ko Lanta, c. 1920. There will probably turn out to be other specimens of japonica in collections. I found one in the TISTR collection collected from Mae Rim, Chiang Mai, in November 1970." That would make the Suan Rot Fai bird about the 5th Thai record...for now.