Knowing the perfect end of fermentation

In the absence of one's hyrdometer (fell and broke) how does one know when the fermentation cycle is complete? When we first started brewing I remember leaving the carboy "cook" until there are no signs of action in the airlock for 5 minutes. I have some concern regarding this method as if left too long does not the yeast die completely and not be responsive to the priming sugar at bottling thus producting a flat (or minimally carbonated) product? I would think that leving some active yeast at bottling would be beneficial for carbonation. Please advise, thanks

For ales, it's not a concern unless you're leaving it for months in secondary.
One rule of thumb is to transfer when the airlock bubbles less than once every two minutes. Leave it in secondary for a week and you shouldn't have to worry about excess yeast in the finished beer.
That's not foolproof, but it makes beer.