There is a sign for everything for people to know it. If you want to know whether a liquid is verjuice, vinegar, water, rose-water, Egyptian–willow water or sallow-water, you can know about it buy smelling or tasting it. If you want to know the name of a metal, you have to assay it with touchstone to see whether it is cooper, gold, brass, platinum, or silver. If you want to know whether fallen body is alive or dead, a mirror or stethoscope will help you.
If you want to know whether a nation is alive or dead, you can know about it through their active presence in the different scenes, and through their intelligence. If you want to know whether a person is a clergyman or businessman, a government employee or a worker, a patient or physician, pious or wrongdoer, a man of this world or a man of the hereafter, there are signs with them which denote their jobs.
Now, if we want to know men of piety, we should get to know them through the features mentioned in the Holy Quran and traditions. When we know them, we can set them as models to be followed, make friend and associate with them.