Depression owns a very wide scale. So many symptoms of other factors and feelings might get confused to depression. For instance, thinking negatively as you said. ((thinking negatively is not necessarily a connection to depression. People can live happy but they have negative based mind which means when something happens they quickly start to comment negatively around the topic)) but in the same time, thinking negatively is one only one of the forms of depression.
Of course there's ways to know and discover it.
Depression has most likely two form, one of the forms is that depression can be described as a state of mind, which the person prison himself inside this hopeless world. Which the only person who can fix this is himself and then the people around the person it's just about recovering the mood. The other form of depression which is the stronger one it's when a huge shock happens, (like the death of the dearest) something out of the line (however this one can be described as the first form as well cause of the person put himself together, he can recover whatsoever) or a physical activity who actually impress the psychological manners. (for instance, why you think the most rate of depressions belongs to teenage period ? Because of the strong hormonal changes and huge activity within the person or so many woman all around the world suffer of depression that hunts them randomly during their menstrual cycle) or when the first form gets too deep and the person get drown into it's depression mood. Which in all these cases medication might be necessary to help the person go through it.
The signs of depression follows as, losing interest of anything, anything that you've been so interested in doesn't interest you anymore and the other things become more meaningless, looking at everything too sadly and feeling sorry for everything, the goals, living, the daily activities everything such as these becomes meaningless and in the eyes of these people the life and it's pyramid lose it's concept.
The saddest part of depression is it's gravity that hunts the person inside, when you start to get depress, the mood gets the person to follow to decorate the feeling, like, listening to sad songs, looking for suicidal aspects and the fear of doing anything that normally you would think it's crazy to do! Will be fade away! You become entirely fearless. And as Gem only can cut gem. Pain can heal the pain into the depression cycle. So it leads the person to the self harm (however not everyone that do the self harm or any kind of cutting is depressed, just to mention)
Depression can happen due a thousand of reasons! So endless to mention! But among the examples i've made, one of the very difficult forms to recover from is when you live in an environment when nobody counts you, nobody would take you serious, everyone doubting your abilities, everyday you repeat your life as "You're stupid, you're useless, you can't do it, you can't do anything right" when the person gets drown in these believes and find the depression, it's very difficult to recover and believe otherwise!
(It's getting too long! I can still talk about examples, forms and the ways of getting out of it and more symptoms and systems of the depression but i guess i said much already!)