INT 2F - DOS Protected-Mode Interface - DETECT MODE
AX = 1686h
Return: AX = 0000h if operating in protected mode under DPMI (INT 31 available)
AX nonzero if in real/V86 mode or no DPMI (INT 31 not available)

See also: 2F1687

2F1686 - DOS Protected-Mode Interface - DETECT MODE