polynomial ring instance check

edit

First of all, do you really need to? Just allow any input, if it does not have the methods you are calling then you will get a suitable AttributeError exception thrown. AKA duck-typing.

If you are sure that you actually have to check the type, use the convenience functions:

sage: from sage.rings.polynomial.polynomial_ring import is_PolynomialRing
sage: is_PolynomialRing('string')
False
sage: from sage.rings.polynomial.multi_polynomial_ring import is_MPolynomialRing
sage: is_MPolynomialRing('string')                                              
False