I would like to test electrical interference in category 5e networking
cables.  We believe we are losing packets due to electrical
interference of nearby wiring.  However, we are not sure how to test
electrical interference.

Please provide an overview of testing electrical interference
including a product recommendation for testing.

Thank you.

http://www.commsplace.com/Knowledge/ITcs/html/tutorials/basic_cable_testing/basic_cable_testing.htm
 basically there are two types of cable testing proceedures. 
1. cable tesing for leakage or high voltage cable tesingt procedures
2. cable test for electromagnetic interference
I belive since u are experiencing packet loss u require
electromagnetic interference tests. the above link is a good source of
such basic tests.
also instrument manufacturers such as fluke / tetronix manufacture
several cable testers.