M.R.
I think what you wanted to know is how do you get a reliable answer from such a young child about illness symptoms? If he does not seem sick, I would not worry too much about knowing every little thing, because you just can't at this age for the most part. If he has pain, he will tell you, and you can observe him and look for signs of illness. As he ages, he will develop the capacity to be more involved in reliable give and take in this way.
Unless he really seems sick, don't sweat it. There are not many serious illnesses that will present without symptoms that you can really see, so you will know when he is really ill, and when he just feels a little icky, it does nto really matter anyway. We all have days when we just don't feel wonderful, but it is not a big deal, and since he does not yet have language for saying that something is not great, but not terrible, you should just assume that some days are better than others, unless you see an outward sign that there is something to worry about. He will be able to tell you one day!
M.