The simple answer...you can't legally unless you ask your subject. However, there may be times where you don't want the subject to know that you're searching for his/her POE, such as in the case of judgment collection or child support. Retaining the services of a license private investigator is always your best legal resource. Docusearch.com offers this service and guarantees the results. http://www.docusearch.com/current-employment-lookup.html
Try to surf the internet and look for site where people are willing to help with your needs. There are some forums where people discuss related issues and giving solutions. Try to look for whatever you want to have for help.