Q:

How to determine whether a timer id is exists?

I could create a new timer with SetTimer(hwnd, id, elpase, proc), but i cannot assure that the timer for the id does not exist.
Is there a way to know whether a timer for the id exist?

A:

I presume you mean SetTimer (Win32 doesn't have a CreateTimer). http://msdn.microsoft.com/en-us/library/windows/desktop/ms644906(v=vs.85).aspx
From the published Win32 API there is no way to find out if a timer for a particular window already exists.
Bear in mind that each Window (hwnd) can have an independent timer with the same id.  I presume you are not in control of the window and therefore don't know which (if any) other timer ids are in use?  
If you are really concerned about overwriting someone else's timer, why not just create a child window within the parent hwnd and create a timer in there?

