如何在asp.net中对复选框进行验证？

<tr>
    <td>
        <asp:CheckBox ID="checkdoc" runat="server" Checked="false" />:Document
    </td>
    <td>
        <asp:CheckBox ID="checktwocheque" runat="server" Checked="false" />:Two Cheques
    </td>
    <td>
        <asp:CheckBox ID="checkIdprf" runat="server" Checked="false" />:ID Proof
    </td>
</tr>
<tr>
    <td>
        <asp:CheckBox ID="checkpancrd" runat="server" Checked="false" />:PAN Card
    </td>
    <td>
        <asp:CheckBox ID="checkAddrssprf" runat="server" Checked="false" />:Address Proof
    </td>
</tr>
<tr>
    <td colspan="4" align="center">
        <asp:Button ID="btnfarmrecordsave" runat="server" Text="Save" OnClientClick="return Validations();" OnClick="btnfarmrecordsave_Click" />
    </td>
</tr>
<tr>
    <td>
        <asp:CheckBox ID="checkdo