Symptoms of Internet Addiction
Because internet addiction is not officially recognized as an addiction by the mental health community, it's difficult to define symptoms with any certainty.
Common sense might have to prevail until further research is conducted. Similar to the use of a drug, if internet usage becomes so time consuming as to interfere with daily life, a problem definitely exists.
Learn More About Internet Addiction Causes and Treatment Options
If someone is missing school or work or neglecting social and family obligations in favor of staying online, then the internet is serving as an unhealthy escape mechanism.
Instead of focusing on "internet addiction," it is probably more sensible to look for an underlying condition such as depression that might cause someone to seek an unhealthy level of escape.
This is not to imply that an internet addiction should be disregarded or lead to self diagnosis. If someone is spending so much time online that it becomes detrimental to their relationships and work life, professional help should be sought to seek an underlying diagnosis.