Check if drop-down box is select.

Check if drop-down box is select.

Hi everybody,
I would like to check if a select option is selected. I mean I have a this html form:

 <form style="background-color:#EEE; padding-left:5px; font-weight:100;" id="presenzeH"
               action="presenzeH.php" name="frmComune" method="post">
  <label id="data">Mese:</label>
    <select name="mese" id="mese" size='1'>
       <option value="0"  style="padding-left:5px;">Seleziona il mese</option>
       <option value="9"  style="padding-left:5px;">Settembre</option>  
       <option value="10" style="padding-left:5px;">Ottobre</option>
       <option value="11" style="padding-left:5px;">Novembre</option>
       <option value="12" style="padding-left:5px;">Dicembre</option>            
       <option value="1"  style="padding-left:5px;">Gennaio </option>  
       <option value="2"  style="padding-left:5px;">Febbraio</option>
       <option value="3"  style="padding-left:5px;">Marzo</option>
       <option value="4"  style="padding-left:5px;">Aprile</option>  
       <option value="5"  style="padding-left:5px;">Maggio</option>
       <option value="6"  style="padding-left:5px;">Giugno</option>
     </select>
<input type="submit" name="invia" value="Conferma"  />
</form>

Befor passing control to presenzeH.php I want to check if user have choosen an option.

Thanks in advance.

Tegatti