...are you asking for real?!?!
There are many, many different options: from the basic 3:15 rule, to the use of nomographs, and there's even an elegant method of telling speed by sound from the hydrophones (allthough I don't think it still works in heavily modded games)
Try a look at this helpful start:
http://www.subsim.com/radioroom/showthread.php?t=96026