The key question to me, is why on earth would you want to know this?
The RDW (Record Descriptor Word) at the front of each record is 4 bytes. The first two bytes are the length of the record. The second two bytes have, AFAIK, 4 possible values:
00 -- record begins and ends in the current block
01 -- record begins but does not end in the current block
02 -- record ends but does not begin in the current block
03 -- record neither begins nor ends in the current block
The 02 and 03 meanings may be swapped; it's been a long time since I looked at the manual that describes these fields.