Help using <cfif> with select option values

I am modifying a receipt screen where I need to make a text box required for certain options.  I need the InstrumentNumber cell required for option values Check and CreditDebit, but I'm really new to ColdFusion programming and I'm not really sure how to do what I need to do.

Any suggestions or help would be greatly appreciated.

<td class="NormalCell">

<select name="Instrument">

<option value="Check">Check</option>

<option value="Cash">Cash</option>

<option value="CreditDebit">Credit/Debit</option>

<option value="Other">Other</option>

</select>

</td>

<td class="DataLabel" style="background: smokewhite;">

<div align="right">Instrument#:</div>

</td>

<td class="NormalCell">

<input type="Text" name="InstrumentNumber" size="12">

</td>