The criteria to recognise a Saint depend on the path that He follows, i.e. the Path of Knowledge, Path of Action, Path of Devotion, etc. Depending on the spiritual level, the criteria can also differ.
At a physical level some pointers are that we can see a glow on the face, the lines on the palm start to become faint, and the fingers and palm become soft and smooth.
At a psychological level the person is able to experience Bliss and His thoughts and behaviour indicate that He is of sāttvik nature. Seekers doing spiritual practice, when in the presence of a Saint, experience Bliss or a thoughtless state. However for an average person this is very difficult to perceive.
According to spiritual science, a seeker of God can be regarded as a Saint only when other Saints acknowledge that He is a Saint. This is as per the law that only a Saint can truly recognise another Saint. A person not of a Saint level cannot proclaim another person as a Saint. This is the best criteria to recognise a Saint.