Be it for yourself, your children, a friend or family member, if you are not a doctor, it is probably best if you don't attempt to diagnose any condition or illness that someone may have, because chances are, you could be very very wrong. Google searches are great if you're
wanting more information on something you already know that you have, but to do a google search for "stomach pain" could do nothing more useful than frighten the life out of you when you discover some of the illnesses and conditions that can cause it!
I've heard adults that simply have no idea how to deal with emotional situations, who would prefer to act out and get away with it, proclaim to have various mental illnesses - bipolar being the most fashionable to get blamed nowdays - as their excuse for acting out; when in actual fact they have never had a diagnosis at all and are just wanting a crutch to explain their inability to face responsibility for their own actions. I've even heard one woman try to use bipolar as an excuse for cheating on her partner! In the end it turned out that she didn't have bipolar at all, although she had psychological issues of a totally different nature. I've known a man to claim he had PTSD as his excuse for being abusive. He didn't have PTSD at all, he was just abusive and looking for an excuse to "permit" him to be so. I've known parents try to claim their child had various behavioural conditions that they were never diagnosed with just because the parent didn't want to accept responsibility for the fact their child was out of their control and misbehaving.
When people do this sort of thing it breeds a resentment in society towards that condition. It doesn't take people long to get very tired of being told that every misbehaving child they come across has ADHD. It then makes it hard for those kids that genuinely do have it, and their parents, to be believed, respected, and accepted. People that do genuinely have a certain condition have enough trouble gaining support without having the ignorance of the majority of the population abused by others in this way.
I have been diagnosed with severe and chronic PTSD. Along the journey to getting that diagnosis correct I had doctors, psychologists and psychiatrists tell me that I could have clinical depression, various anxiety disorders, schitzophrenia, bipolar, multiple personality disorder, post natal depression, an inability to deal with reality, and a host of other things I can't even remember the names of!
Mental illness is definately one thing that should never be self-diagnosed, as the subject is so complicated that even the professionals can get it wrong. But mental illness isn't just the only thing we should avoid self-diagnosing. Many physical illnesses can have similar symptoms as well. Measles can easily be mistaken for chicken pox. A cold could be hayfever. Gastro could be food poisoning. A heart attack could be a nasty anxiety attack, or angina. Lower back pain could be a spinal problem, muscular problem, or a kidney problem. A lung infection could be lung cancer. Unless you have been to see a doctor, how are you going to know for sure?
ADD, ADHD, Autism, Aspergers, OCD, ODD.. all these conditions, and more, have some similar traits in common, and its not that uncommon for someone that has one of these conditions to also have another. There are so many illnesses and conditions in the world that many of them are very easily mistaken.
The other problem with trying to self-diagnose is that humans naturally suffer a certain level of paranoia. Some of us have it worse than others, and it can lead to serious anxiety problems. If you have an ache in your leg and you try to surf the internet to find out what could be causing it, the simple strained muscle that is causing your pain can suddenly have you panicking thinking that it's a blood clot, cancer, or any other numerous things that could cause pain in your leg. If you get yourself all wound up about it, you could end up having an anxiety attack, which could then have you believing that you're about to drop dead from a heart attack. The more you worry, the worse it gets, and eventually the stress you are putting yourself under can end up making you very physically ill for real.
On the opposite extreme, you might think you have a common migraine that won't go away, when in actual fact you have bleeding in the brain that is going to be the death of you. If you assume that it's something minor and don't worry about it, it could end up being too late to save your life if it's not.
I think I've just given enough good reasons why going to a doctor about something that doesn't seem quite right is a far better way of finding out what the problem is than turning to Dr Google Search for the answers. As I said, Google is great for getting more information once you know what you're looking for, but it's not advisable to use it for a self-diagnosis.