Hi Neil,
Thank you for your interest in Syncfusion Products.
We do not have CausesValidation property for the toolbar items. But you can use Page.Validate() method to Validate Page in toolbar item click event. Please refer the sample in the below link which illustrates the above:
http://files.syncfusion.com/support/samples/Tools.Web/7.1.0.21/F80431_ToolbarValidator/main.htmPlease try this and let me know if this helps.
Thanks,
Melba