I was unsure about the mantra cry too, but I soon worked out the difference between that and when he needed me.
My DS just makes a monotone 'waaah' noise. Sometimes he yells out in short bursts, and we do get the occasional squeal, but there's definitely an obvious change when he needs me. It becomes more urgent and frantic-sounding. You'll know when it happens, I'm sure.
DH isn't so good at deciphering yet though... He thinks any increase in volume is a real cry
but he's learning!