DosQueryThreadInfo will return information about threads for the specified process or task.

#define INCL_WPOS
#include <os2.h>

ULONG     ThreadID;
ULONG     ID;
ULONG     InfoLevel;
PVOID     InfoBuffer;
PULONG    InfoBufferLength;
PULONG    InfoEntryCount;
APIRET    ulrc;              /*  Return code. */

ulrc = DosQueryThreadInfo(ThreadID, ID, InfoLevel,
         InfoBuffer, InfoBufferLength, InfoEntryCount);