I notice my ds is different when I compare him to other children his age, but at first it was the HV, then at his SN nursery (he ended up going there, when the nursery he was going to, asked me to stop bringing him because they couldn't handle him, his screaming non-compliance, I was still oblivious then), that was when he was 2.5, I think...
we have never "pursued a diagnosis", we are (I think) being gently urged to consider one... all the caring professionals look at us with weary disbelief or have done... when ds started ms nursery at 3.3, we were called to a meeting after the first week, they could not cope with him, since then (he is nearly 4 ) we have just tried to be open to the help the professionals have sent our way
my dh claims he was "odd" as a child, that ds is just odd too, but I am on the fence/aware that the head in the sand approach doesn't really work!
at home my ds can seem almost normal, but then that is our own idea of "normal" i.e we are doing okay, his meltdowns and lack of communication are hard to deal with but not in a huge way.... he sleeps (well in the last 6 months he hasn't slept well) , eats, plays, cuddles, laughs, is generally a sweet, loving wee boy....
but at nursery he is not like the other children, he stands out, is obviously different, to the other children too, just waiting to go into nursery this is obvious, never mind when they have little show for the parents...
I suppose we do have behavioural problems, my ds language is delayed and, echolalia has been evident, though he seems to be getting past that stage, starting to build on the language he has, ds has sensory stuff going on too, stimming and other wee quirky behaviours.
I often think my dh only sees ds in the home and rarely with his peers, when I meet other children my ds's age I am always comparing them, amazed at what other 3+ yr olds are like, thinking what his older sister was like at that age, etc... although I always remember reading that it is wrong to compare your child, hv always used to tell me that too, when I had my PFB!
I still don't feel ready for a diagnosis, still cling to every little positive thing ds does that might be proof of his "normality" but I know he is not developing in the NT way...
sorry for the long winded response! I suppose it is a slow dawning realisation for me, and my dh is lagging behind, but if you asked parents at nursery if my child was different, I am pretty sure they would all say yes.