INT 15 - DESQview - QEXT.SYS - INSTALLATION CHECK
AX = 11DEh
Return: CF clear if installed
AX = segment at which QEXT.SYS is located
Note:   a private entry point may be found by searching the beginning of the
returned segment for the signature string
"QUARTERDECK EXTENDED MEMORY MANAGER 286"; the word immediately
prior to the signature contains the QEXT version number in BCD,
and the word prior to that contains the offset within the QEXT
code segment of the private entry point

Call private entry point with:
AH = 00h ???
AH = nonzero ???

See also: 673F

1511DE - DESQview - QEXT.SYS - INSTALLATION CHECK