use something like the following that returns an array of the number of words per line in your textfield:
function lineBreaksF(tf:TextField,s:String):Array{
var returnA:Array = [];
var textA:Array = s.split(' ');
var saveS:String = tf.text;
tf.text = '';
var num_line = 1;
for(var i:int=0;i<textA.length;i++){
tf.appendText(textA[i]+' ');
if(tf.numLines!=num_line){
returnA.push(i);
num_line=tf.numLines
}
}
tf.text = saveS;
return returnA;
}