Mental health experts are worried: Depression is on the rise in the US, especially in teenagers. From 2005 to 2015, depression rates in kids between the ages of 12 to 17 spiked, and the suicide rate for teenage girls is now the highest it's been in four decades.

Experts aren't sure what's causing the disturbing trend, but they're concerned about the mental health of the country.

In New York City, coaches are working to recruit and train a kind of civilian mental-health army of 250,000 volunteers. The hope is that people trained in "mental health first aid" will be better able to start conversations, lend a helping hand, and share compassion for friends, colleagues, neighbors, and other fellow New Yorkers who are dealing with mental health issues.

New York residents who volunteer get a free, day-long training — I recently participated in one. My fellow volunteers and I were taught some simple ways to spot the difference between someone experiencing a bad mood or a few bad days and a person with more serious, long-term depression.

If you're worried that you or someone you care about may be suffering from depression, but you're not quite sure how serious it is, ask yourself how the person is doing when it comes to four key happiness measurements.

Is you/your loved one's mental state having an impact on the person's ability to:

If these four pillars of life are feeling compromised by a persistent cloud of sorrow or indifference, it may be a sign that depression is at hand.

The World Health Organization (WHO) estimates that around 5.9% of the US population is dealing with a diagnosable depressive disorder. That means more than 1 in every 20 Americans are suffering from depression at any given time.

Depression is the single largest contributor to "non-fatal health loss" worldwide, according to the WHO. It affects around 4.4% of the globe, though those rates vary from place to place. Some of the highest depression rates in the world are among women in Africa, while some of the lowest are in men in the Western Pacific islands, the WHO says.

When health care professionals in the US diagnose depression, they use a manual called the DSM-5, or "

Only a trained mental health professional can officially diagnose depression, but when it comes to on-the-fly "first aid" for depression, these tell-tale signs can be a first warning that it might be good to ask for help or see a counselor.

In New York, you can contact health care professionals at this link. They're available by text message or online chat there as well.

If you're having suicidal thoughts, call the National Suicide Prevention Lifeline at 1-800-273-8255.

There are also new apps coming online that aim to provide some of the benefits of therapy on the go. These include Woebot, a texting-based system, but they're untested and by no means a substitute for a mental health professional.

The National Institute of Mental Health also offers a full list of resources for depression on its website.