St John of the Cross offers three criteria for determining whether someone has entered the dark night of the senses. The experience is something distinct from the effects of depression, melancholy, or sloth.
In brief, here are the three indications of the dark night of the senses:
Stanzas of the Soul
On a dark night,
Kindled in love with yearnings
—oh, happy chance!—
I went forth without being observed,
My house being now at rest. 
"These times of aridity, then, cause the soul to journey in all purity in the love of God, since it is no longer influenced in its actions by the pleasure and sweetness of the actions themselves, as perchance it was when it experienced sweetness, but only by a desire to please God. It becomes neither presumptuous nor self-satisfied, as perchance it was wont to become in the time of its prosperity, but fearful and timid with regard to itself, finding in itself no satisfaction whatsoever; and herein consists that holy fear which preserves and increases the virtues."
Oh, happy chance!
When God leads the soul into this night of sense in order to purge the sense of its lower part and to subdue it, unite it and bring it into conformity with the spirit, by setting it in darkness and causing it to cease from meditation (as He afterwards does in order to purify the spirit to unite it with God, as we shall afterwards say), He brings it into the night of the spirit, and (although it appears not so to it) the soul gains so many benefits that it holds it to be a happy chance to have escaped from the bonds and restrictions of the senses of or its lower self, by means of this night aforesaid; and utters the present line, namely: Oh, happy chance! With respect to this, it behoves us here to note the benefits which the soul finds in this night, and because of which it considers it a happy chance to have passed through it; all of which benefits the soul includes in the next line, namely:
I went forth without being observed
This going forth is understood of the subjection to its sensual part which the soul suffered when it sought God through operations so weak, so limited and so defective as are those of this lower part; for at every step it stumbled into numerous imperfections and ignorances, as we have noted above in writing of the seven capital sins. From all these it is freed when this night quenches within it all pleasures, whether from above or from below, and makes all meditation darkness to it, and grants it other innumerable blessings in the acquirement of the virtues, as we shall now show."
 Handbook to the Life and Times of Saint Teresa and Saint John of the Cross, London, Burns Oates and Washbourne, 1953, p.18.
 Saint John of the Cross, Dark Night of the Soul, chapter XIII, §12
 Saint John of the Cross, Dark Night of the Soul, chapter XI §3