I have to say that even though I may not say something my face often gives me away with what I am thinking. The other day I was at a new playgroup (everyone else is established friends) and I really disagreed with what another mother was doing. (Her toddler did something minor wrong - she grabbed another kids toy - and so was totally rejected by the mum. No eye contact, no touch) The toddler was so distressed in the end. I could tell my face gave me away and the mum spent the next half an hour talking about other parents that judge. But honestly I don't feel capable of reacting in any other way to that situation.
I guess it is human nature to constantly assess situations and if you are discussing how you parent then you do open yourself up to other peoples POV. But I like Fionas advice on how to handle it. After all, if they think you are struggling then they will offer their solutions. If on the other hand they seem to be putting you down because of your choices then it may be time to be firm.