writing extension -- custom type validation

[Eric Hagemann]

I am making a new data type in an extension module.

Is there as way to detect  / check for a custom type (of the type I am
creating) from a PyObject ?

As an example I can see if an object is a PyInt by using
PyInt_Check(PyObject *obj)

The crux of my question comes to the initialization function.  I want to
"overload" the input type.  It is valid to initialize from a string,
sequence of ints or another custom type object -- what I am looking for is a
positive confirmation of the custom type.

My eyes are going googly from the docs -- did I miss something obvious ?

Cheers
Eric Hagemann