The correct thing to do is also to check the carry and overflow flags after (I)MUL, which does the right thing even when only the lower bits of the multiplication are saved.

Here's an example where there is overflow, but your suggestion would not catch it:

    mov eax, 0x7fffffff
    mov ecx, 2
    imul ecx ; edx:eax = eax * ecx
    ; edx = 0
    ; eax = 0xfffffffe
    ; overflow flag = 1

There is signed overflow here, but testing edx (the higher word of the result) would not catch it.