You can use the ASP.NET required field  validator control  for this. Simply
drag one RequiredFieldValidator control from the SPD toolbox for each field
to be validated. Then using the properties panel for each control you can
point one at each field to be validated. The ControlToValidate property
entry should be in the form  of 'ff#_#' for example ff3_1 for the third form
field in the web part. If you highlight the form field that you want to
validate you will see the form field ID in the properties of the field. I
do this all the time in SPD on custom  pages where I've added a DataForm Web
part.

Why do you think you can't use the requiredFieldValidator control to
validate individual field controls.

Off the top of your head can this be used for conditional validation? I
have checkboxes on a NewForm.aspx (and EditForm.aspx) that, when checked, show
rows below the checkboxes and the rows become required. Only thing is that I
can't conditionally validate within SharePoint's controls. But, if I used the
RFV example you discuss then how would it be possible to conditionally validate
the fields?

I don't think you're going to be able to do that with these controls without
the use of code (scripting) which isn't supported in SPD.

Are the other ASP.NET validator controls supported? Seems like you could
write some decent client validation with a CustomValidator -- and since
SharePoint isn't going anywhere without client scripting, you might be able
to make that fly.

Because of its reliance on custom  server side code the Custom Validator is
the only Validator that won’t work in SPD as far as I know.

Custom validator will work in SPD. I am using it. I was asking for
RequiredFieldValidator.

I took another look, and if you use the CustomValidator the way it is
implemented in SPD then it will work. The problem is that the
CustomValidator in ASP.NET is normally used with both server side code and
has the option to add client side JavaScript. The Javascript will work, but
server side code can't be added in SPD. If you drag the CustomValidator
from the toolbox in SPD it masks the properties associated with server side
code. As long as you stick to javascript it will work.

As I pointed out the RequiredFieldValidator won't work with certain field
types like People/Groups and Lookup. You may be able to write some
Javascript for the CustomValidator that will work similarly.

I don't deny your point. It does work. But for fields  of type Person
or Group or Lookup, if I follow your way, it says "An unexpected error has
occurred". Thats why I raised this question.

That explains the problem. The issue is not the ID of the control. It’s
that the built in ASP.net validators are all built around an expectation
that they are validating text. Person or Group fields  and Lookup fields
don’t result in text, so an error is generated when you try to use them with
these types  of fields. You would need to either write your own client side
Javascript or make the underlying field  required to make these fields
required. You can’t use the validator controls with these kinds of fields.
For what you want to do I think you will need your own custom  Javascript.

Is this veracious? We can use RequiredFieldValidators with ListBox,
DropDownList and so on. I believe the validators are not expecting a text in
this case, if I am not wrong. Moreover, I could very well use
<asp:CustomValidator> (which is also an ASP.Net validator) with the Person
or Group field. Please try to understand that I am not trying to argue with
you, as you are a highly experienced and knowledgeable architect and I
am just a learning developer. I am trying to understand the concepts, thats
all.

SharePoint allows you to create custom  web parts to help you provide highly customized solutions to your clients/users. The properties of these web parts are added by default but to further customize the solution, you can also create custom properties. The following example demonstrates how to add a custom dropdown property  to a custom webpart.

// Create a Enum for Languages

public enum LanguageByEnum

{
English = 0,
French,
Spanish
};

Thanks,
Bijayani
www.mindfiresolutions.com/Sharepoint-Support.htm