There is no way to know "for sure" if someone is going to be a school shooter, or a beauty pageant winner, but there are indicators of the liklihood for both.
While society pays attention to the indicators of someone likely to become a pageant winner (possibly because the indicators are obvious?) there is less attention paid to the indicators that someone may become violent in any way. IMHO that is because people want to avoid admitting that violence exists and especially that it exists right in front of their face. While there are literally hundreds of book and articles that have been written about recognizing the potential for violence, and especially the potential for imminent violence, all directed at general audiences as opposed to mental health specialists, they are either not well known or are simply ignored.
Go back and look at the accounts of all the school shooters - the warning signs were all there. In the Va Tech case someone actually tried to "do something" based on the warning signs. Sadly, what little was done failed to connect all of the available dots and there was no follow-through once someone passed the problem off to someone else.
We do regular diagnostic checkups of our automobiles (or should) and attend to any problems as they are discovered instead of waiting for a catastrophic failure of a known problem part. We do not do that as well when it comes to people and their behaviors - partly because of an appropriate dislike of infringing on the right to, among other things, be crazy as a bat. However, it is no infringement on another's right to take steps to reduce one's exposure to harm should the crazy person suddenly explode into a killing frenzy - by either not being around the crazy person or, since we cannot know every crazy person, by having a means of defending against their killing spree once it starts. Guns are more efficient and effective than baseball bats, but baseball bats should not be ruled out just because of that. The choice to carry openly or discreetely is a personal one. So long as you do not infringe on another person's choice, you will get no complaint from me about how you carry - even though your active support for OC will be sadly missed.