Require a checkbox to be checked:

<asp:CustomValidator runat="server" ID="rfvMember" OnServerValidate="cbMember_Validate" ValidateEmptyText="true" ErrorMessage="Member coverage is required." Display="None"></asp:CustomValidator>


protected void cbMember_Validate(object sender, ServerValidateEventArgs e)
	{
		e.IsValid = cbMember.Checked;
	}

source

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