Even in laughter the heart may sorrow, And the end of mirth may be grief. (Proverbs 14:13)
You can't tell a book by its cover, especially if the book is a person. We must learn to be sensitive to the prompting of the Holy Spirit with regard to what we see and hear. For what we see and hear is not what is always being communicated. It is not a matter of always having to judge people, but it is a matter of discerning of spirits. Laughter and merriment at an inappropriate time most likely is reflecting hidden hurts or sorrows. We need not be paranoid or always attempting to read into someone something that is not there, but we are to be receptive to the prompting of the Spirit of God. It is not to be mystical, but supernaturally natural. We are spiritual beings; therefore we are to likewise be spiritually discerning. Not everyone will have this ability. However it is a Spiritual gift worth seeking, especially if you are in the people business.