To my knowledge, you’ll just have to look for the errors.
If a send timed out, the $send($nick).cps and $send($nick).pc will be zero ($send($nick).secs will return the DCC connect time out amount, but the user could change this via mIRC Options).
Use on QUIT or on PART to be more specific will error messages if you want (User quit IRC etc..). In general, anything else than a time out mIRC just displays failed.