Don't worry, it does get easier to spot those cues!!
I had awful trouble getting it right with my ds, sometimes you have to be super observant. It can often be the smallest of signs and they're very easy to miss. Just persevere, ingnore unwanted comments and know that you are connecting with your baby.
As your son gets older you will be able to spot the signs before they happen, you just have to have faith in yourself!!
I know how annoying it is when people make comments that contradict your decisions, i get it all the time! :roll:
I either tell them straight or just ignore them, depends what mood i'm in!