Look at the setting and the character's state of mind, and then think about what you want the reader to see. Is there a descriptive symbol or two that works naturally within the scene to help foreshadow an event or theme, or create insight into the character's emotional plight?
A lone tree on a hill
An animal driven away from the herd/family group
A single bird flying without a flock (geese, for example)
A lone wolf
A falcon or eagle's nest at the top of a tree
Contamination units in hospital
Victims of abuse
Victims of alcoholism or drug abuse
'Corner Kids' (kids in schools who are not accepted by others)
Solitary confinement (prison)
Adrift on a raft
Alone on a boat
A teen with headphones in
Fishing shelter on a lake
Being snowed in
These are just a few examples of things one might associate with Isolation. Some are more powerful than others. A snow-shrouded cabin in the woods is a strong symbol, and likely will not require reinforcement. However, a lone wolf may not foreshadow isolation on its own. Let the story's tone decide if one strong symbol or several smaller ones work the best.
Closely related to Isolation: Alienation.