Validate Devexpress controls at client-side using javascript

In order to validate devexpress controls in the page using javascript, use the following code:

ASPxClientEdit.ValidateGroup(null); or ASPxClientEdit.ValidateGroup('validationgroup');

This method returns true if all controls on the current page with the specified validationgroup pass validation and false if they don't

Example:

<dx:ASPxButton ID="btnAdd" runat="server" Text="Add" AutoPostBack="false">
     <ClientSideEvents Click="function(s,e){ if(ASPxClientEdit.ValidateGroup(null)) { alert('Validation OK'); } }" />
</dx:ASPxButton>

or using a normal button

<asp:Button ID="btnAdd" runat="server" Text="Add" OnClientClick="if(ASPxClientEdit.ValidateGroup(null)) { alert('Validation OK'); return false; } " />

Don't forget to use return false; OR e.processOnServer=false;  OR AutoPostBack=false; if you do not want the button to cause a postback.

Check Online Documentation for more details