Microsoft Cab File Header is simple..
MSCF followed by 4 zeroes
4D 54 43 46 30 30 30 30
InstallShield files (cab and hdr) have ISc( as a tag
49 53 63 28
if the above 4 bytes match, then check the DWORD at file offset 0x14.. if it is the actual file size, its a hdr file,
if not, its an installshield cab file
*edited* had the ISc( the wrong way round, because i viewed it as dword view...