How can I test if a text box exists on a web page?

I am trying to set focus to a text box when my page loads. The ONLOAD calls this line.

document.MYFORMNAME['txt_part'].focus();

This works great!

However, I build my form on the fly and sometimes my text box is not there. If it is not there, then the above line displays an error message.

Is there a way to test if the text box exists before trying to set focus?

Thanks,

Jerryt