How do you know if Someone is strong?

This depends entirely upon what you mean by "strong." If you mean physically, then they can do things that weak people cannot, like lift heavier weight or move heavier objects. If you mean mentally, then they are more mature and able to handle more responsibility and stress than an immature or weaker person would. If you mean emotionally, they can deal with emotional stress better, and if you mean spiritually, then their faith is not challenged by what happens to them.