Hi Madhukar,
Try SPRO -> CRM -> CRM Cross-Application Components -> Generic Interaction Layer/Object Layer -> Basic Settings.
in that every component there is implementation class for that object..
if you're talking about GenIL Handler Concept (which was introduced as of CRM 7.0 Ehp1 SP02) then you can find them by going to tx. GENIL_MODEL_EDITOR, clicking on the desired object and look at Implementation Settings section. There is a field Handler Class . But I've just checked on our system: no standard objects have handler class at all (table WCF_GIL_OBJECTS field HANDLER_CLASS).
If you refer this document you'll find that you can implement an object using only Implementation class. And I believe most of standard objects using this logic so far. So if you'd like to discover which class handles which objects you should start from the Implementation class of the component and dive deeper in it.
hope this will help full.
Thanks & Regards,
Srinivas.