This paper describes an approach to discover and verify two fundamental relationships in web service message exchange flow patterns, i.e. support and correlate, based on the abstract WSDL specification of the service operations. Compared to methods used in previous studies, this approach is a significant improvement in that flow chain relations can be discovered and identified based on flexible and inexact match of XML Schemas using differential information in plain WSDL and XML Schema files. This approach is also efficient and extensible as it is based on a tree model of schemas and modular node equivalence rules. Pilot experiments on some web services standards, e.g. WS-Session, WSEventing, Parley X, etc., indicated that the proposed method is accurate and effective in identifying the message flow dependencies and related issues in service descriptions. The derived flow pattern relations can be represented as Petri nets that have some practical applications for web service verification, implementation and consumption.