<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;
	}

NOTE:** Can't put controltovalidate property on customvalidator for this or it'll break since checkboxes are one of the few controls that .net can't validate.

Require a checkbox to be checked:

C#