The following contribution to this subject are personal thoughts/insights only and are only intended to offer things to consider before discussing your concerns with a professional.

Sometimes what looks like depression to other people may not, in fact, be depression at all.

There are times when people are unhappy (which is a different thing from depression). There is also a condition, adrenal fatigue, which is sometimes misdiagnosed as depression (even by professionals).

Under stress, the adrenal gland is responsible for a number of changes, including elevated stress hormones. The changes that take place under stress make it possible for a person to "keep going" under stress.

Adrenal fatigue occurs when a person has lived under long-term stress for too long, and his adrenal gland "runs out of juice". Symptoms include difficulty concentrating, craving salt or sugar, feeling depleted (rather than invigorated) after exercise, and other symptoms which can mimic depression.

There is no treatment for mild and moderate cases of adrenal fatigue. There are hormone treatments for severe cases (Addison's Disease).

Even without any adrenal gland insufficiency, however, stress is exhausting. Too much stress can "be depressing" without the presence of clinical depression. People who are unhappy feel stressed because of being unhappy, so that factors in as well.

There are other causes of appearing depressed as well. Someone who sticks to a rigid diet may lack energy (or even the ability to function intellectually or physically) as a result of not getting enough of the foods or nutrients needed to function optimally.

Medical conditions other than depression can also contribute to a person's feeling depressed. People with heart problems (particularly those who had heart attacks) are often depressed following the episode.

Also, sometimes a person who suffered a loss has not seemed to get over it as quickly as those around him think he should have. Severe loss can take far longer to process than observers who haven't gone through that particular type of loss (or who weren't as emotionally invested prior to it) sometimes think it should.

Elderly people often have depression as a result of medical conditions, feeling "too tired be bothered" with some things, or living through a lot of recent loss and not having had the time to get over it.

Even when people do have clinical depression there are those who prefer to try to work their way through it themselves. Some people do not want or need to "talk things over" with a professional. These are people for whom "re-hashing" problems isn't what they want to do. Many people are actually afraid of side-effects of medications that are so freely prescribed today; so many prefer to try to go as long as possible without resorting to them.

In other words, a lot of things can appear to be clinical depression when, in reality, they are either something else or have a cause that has rooted somewhere other than in their mind.

It isn't easy for family members or friends to know what to do when someone else seems miserable and depressed, but the fact is a lot of unhappiness is not clinical depression at all. (It has recently been found that doctors often treat people who are "just unhappy" as if they have clinical depression; and, again, some people with adrenal fatigue are misdiagnosed as having depression.)

We are all told that we should never ignore the signs of depression in loved ones, and we're told that we should urge them to get help. It is frightening for family members to worry about the possible consequences of depression, but sometimes even people who have depression have enough will to live to keep on going. The awful thing is we, the observers, never know for sure if it is depression or how much we really do need to worry. A complicating factor is that professionals are so ready to just prescribe anti-depressants without asking about any possible roots of that depression, or whether it is, in fact, "real" depression at all.

There is no simple answer to your question, other than trying to communicate with him in a way that let's him know you're open to the idea that he may not have depression at all, but may have a medical condition (even if it's adrenal fatigue) that is making him feel as bad as he appears to be feeling. When someone appears miserable there is sometimes nothing worse than having a family "on their back" about it, particularly when the person, himself, may know he has his own reasons for being unhappy; and those reasons are not clinical depression.

Sometimes a person who won't discuss apparent depression with one family member may actually talk it all out with someone else. Asking your father if he has someone with whom he does talk openly could ease your concerns.

Learning as much about not only depression, but about all the other conditions that can mimic it (including "plain old unhappiness") is something you could do. Talking with someone who has experience/expertise in dealing with people in your situation could be helpful.

If your father isn't aware of all the medical conditions, and stress situations, that can appear to be depression (or cause it) it could be helpful to make him aware of them. Once you know he has a good picture of all the things that could be at the root of his problem, ask him to at least talk to his physician about his situation. Sometimes a physician can get a reading on whether a person is really depressed or not. Sometimes people will listen to their own doctor.

One of the most difficult things to know is whether someone is unhappy and stressed out (and maybe even depressed) but solid enough to keep going, whether he's someone in potential danger as a result of severe depression, or even whether he's someone who is fine on the inside but too tired to be bothered trying to be cheerful for the benefits of others. To further complicate things, there are even people who will act depressed around a family member because they believe people should not have to pretend to be happy when they're with close family or friends.

Whether or not your father has clinical depression is not something you can know if he has not had it diagnosed by a competent professional (who doesn't jump to the conclusions that "everything is depression").

It would be probably be wise for you to learn everything you can about the signs that a person may "do something drastic", and if your father truly demonstrates those signs talk to a professional about what you think you're observing.

Other than that, maybe the wisest step would be to talk to your father about your concerns, about all the possible causes of the signs of depression that he's apparently showing, and asking him if there are things he's going through that you don't understand and that he's can't discuss with you. If he says he has things he can handle, ask him to assure you that if he gets to where he feels he can't handle them he'll at least talk to his doctor (or a doctor) about them.

Keeping the conversation as low-key as possible may help keep the lines of communication open. If the topic turns too heated or makes him feel you don't get him credit for being a solid adult he may stop trying to communicate with you about it at all.