Practical model validation techniques are needed for model driven development (MDD) techniques to succeed. This paper presents an approach to generating inputs to test UML design models that are produced in the detailed design phase of an MDD project. A symbolic execution based approach is used to derive test input constraints from the paths of a Variable Assignment Graph, which integrates information from UML class and sequence diagrams. The constraints are solved using Alloy, a configuration constraint solver, to obtain the test inputs. The results of a pilot study carried out to explore the fault detection capability of the test inputs are reported.