Is there a consensus on how to check a polymorphic instance?

[Mike Meng]

hi Steve,
Your example is helpful. Thank you.

Here is my problem. My project needs to parse some user input
string according to  specific rules. The rules can be defined with a
string, a CSV file, an XML file, and other forms. Followed the standard
OO style,  I designed a dictionary-like base class to abstract those
different sources and provides other parts a uniform interface to query
those rules.  I put some hard code into the base class and expect user
to inherit from it.

    Any good suggestion?

Mike