Where is the line?

I took one of my kids to the doctor the other day for a hearing exam.  We've been having issues with him for awhile with him not seeming to hear us.  We ask him to do something and it is either "what, what, whaaaaaaat?" a million times or he acts as if no one has spoken and goes on about his business. 
The funny thing to me though it is not constant.  Sometimes he seems to hear just fine especially when dessert is mentioned :)  But, there are times when he just doesn't hear what we are saying and he was getting really upset and I was getting frustrated.   While weighing the decision of whether or not to call and make an appointment I started thinking back realizing that other people have mentioned it.   So I felt justified in taking him to the doctor because surely if other people have mentioned it then I am more than just a paranoid mom right?
So I took him and currently he seems fine.  Which is fantastic but then the other edge of the sword gets you and you think so he is just intentionaly ignoring me?  Not so fantastic. 
According to the doctor though a very common thing in children is fluid building up on the ears and when that happens they do not hear right... it is like cotton being stuffed in their ears & other than not hearing you there are no clear symptoms like pain, etc.    I was hugely relieved when he mentioned this because it made me feel less like a crazy mom.  So we follow up in a few months and if he has a bad day I take him in.
It raised a few questions in my mind though.  Where is the line?  You know the line that is there between being paranoid and having a legitimate concern.  Do others have to weigh in before it is enough "evidence" to do something about the problem.  What do you think?