Like you...when it frays or looks all gacked up and twisty!

Be checking for any nick, rub, abrasion, curly que etc, constantly and especially after fighting a fish. 

Also remember that there is wear and tear on the knot at all time even when there is no visible or tactal evidence of problems up from the knot.

In other words, get in the habit of retieing, because most lost fish can be attributed to the line separating.

I retie often and my experience is not losing many fish because of the line breaking.

jmo
BillB

As most that have fished with me would attest, I tend to be on the cautious side when tying knots  (I don't want to hear it! Ex-Prez and others)

Even when you don't see a knick in your line, give the knot a good yank after you catch a fish or hang up. Sometimes you'll be surprised to find that it was weaker than you thought... If the knot holds up and you don't have any knicks, then you are good to go!

Chris