your car will have little wires in the pads. when the pad wears down to the point that the wire touches the disc it makes and electrical circuit. a little light comes on on the dash display every time you brake. this tells you that they need renewing . don't panic as soon as they come on, you have a small in built safety margin so you can get it done soon rather that immediately. DON'T LEAVE IT FOR TOO LONG THOUGH!!
you can rely on the warning lights thats what they are for.