There are several ways this can be done. You will first have to try all the personal identifiers. Personal identifiers are the schools, previous addresses, universities attended, family members? details, professional details and such details. Armed with as much information as you can, you venture into performing online.
There are several white pages services online where you can perform different kinds of searches, such as the city and state search, city only search or state only search. You may even want to try the last name searches, and these can be especially useful if the person has a rare and unique surname. You may even be able to identify the person's relatives with the same surname.
If all these searches do not yield any results, then you can use one of the paid people search services. If you only have the phone number, you can also try using the numerous reverse lookup services that can help you identify the person belonging to that phone number.
You can also use the search engines like Google and Yahoo to search for people. You can filter your research and get more accurate results by using personal identifiers.
There are many online communities and social networking sites where you can perform a people search. These places are goldmines to find information on people as all sorts of people in thousands visit these . Many people leave their personal details on their profiles and at other places, making it easier to perform a people search here and come up with results.
If all the free options do not yield results, then there are the paid people search services you can use and these are the best in terms of accuracy and ease of use.