Body Language that shows Loneliness

Observation is the key to writing descriptions and interpreting the signals given by the body. When a character feels lonely there is usually a pattern in his/her body signals. Of course, every character is unique, however, the pointers below can be of help to trigger writers' imagination.

Lonely people are generally disinterested in their appearance
Voice may end in monotone
May keep their schedules tight
Often sigh heavily
Feel happy about receiving mail
Rush to check phone alerts
Hug oneself
Make small talk with electrician, plumber, bus driver, etc
They think they are introverts
May call family and friends often
Daydream
Sometimes feel unworthy.

Characters can manifest any of these signs depending on what, as a writer, you wants to achieve at that point in the story.

Finally, search the web for  'loneliness' in images to help identify how lonely the character feels.