How to Validate before Deployment:

The Setup client now performs error checking before every deployment.  The validate button must be selected first before deploying any changes.  This helps to ensure that there are no errors in the changes of the setup before rolling out to production.

When making any changes within the Setup client, the validate icon will become highlighted.

Once you have finished making any changes, select the validate button.  If no errors are returned, the validate button will become grayed out and you can select the Deploy icon to deploy your changes.

If the changes come back with errors, you will see a list of all invalid changes in the Invalid drop down.  Hovering over each invalid will give you a summary of what and where the error is located.  By clicking on the actual invalid error within the drop down will take you directly to the error to be fixed.  Once you have made the necessary changes, the Validate icon will becoming highlighted again.  Select the validate icon to deploy.