How to identify the health condition of an electric motor by trendind the difference between the magnitude of line frequency and the magnitude of the side band?
I heard there are some standard values published.
Any IDEA?

I assume you are talking about current signature analysis.

As a rough thumbrule:
53db difference - alert, 45db alarm1 40db alarm2.

http://www.compsys.com/enews/knewspro.nsf/v/TBUT-54WLKJ

Knowing the speed of the machine and number of rotor bars and load at time of the test (prefer above 75%), you can develop finer limits geared toward the motor and test conditions. There may be other things you want to look at also - swirl effect near 5*LF.