Description, history. And excluding other conditions that might mimic mania--sometimes when it isn't suspected it is precipitated by antidepressant medication.
Mania. Mania, and bipolar disorder from whence it comes are not really tested for, so much as evaluated for by a psychiatrist. That being said, a personality test known as the MMPI-2 (Minnesota Multiphasic Personality Inventory 2nd Edition) can pick up on manic tendencies as well as a host of other mental phenomena.
Mania. Thorough history, mental status exam, & workup to rule out organic factors.
No tests, must... Review history of moods, patterns of thinking and behavior carefully, for example, a two-week long episode during which one rarely slept, felt euphoric, though easily irritated, flooded with creative ideas (that might later prove not so creative--like brilliant insights in dreams that vanish on awakening) read meaning into incidental events and took increased risk-physically, financially, sexually.