Attach a Modal analysis to the Static Structural model of the surface. Apply the same constraints to the Modal analysis. Solve the Modal and see if you have almost zero natrual frequencies. Watch the animations of those to determine which surface is not connected.

Open the Surface geometry in SpaceClaim and on the Workbench tab, click the Share button. This will cause all coincident geometry to share nodes. Then rerun the Modal and you should get a non-zero first mode frequency.