It can be quite difficult to actually spot someone who is suffering from secret loneliness. Its very perceiving indeed but if observe and taken extra attention to it, you are likely to spot the link, that if you know the symptom. Lonely people often display anxiety, depression and all kinds of ill health complaints such as headache, insomnia, exhausion(mentally n physically), loss of appetite and body pains. But do take note that they do not express their concern nor they realise about this problem that lies in them. Others who knew that they are suffering from it do not really tell in a short sentence or word. They will describe something which can be link or be implied to loneliness. While others may refuse to admit that they are lonely which can lead to distortion. Therefore its good that you try to open up if ur friend who is having issue with loneliness. And attend to them physically and psychologically.