What is the proper way to test if a checkbox field is set? 

The html is <input type="checkbox" name="cb_phonelist"
value="phonelist" checked>

When I try cgi["cb_phonelist"], I get a zero-length array no matter
the state of the checkbox. Same for cgi["phonelist"]. Same for
cgi.params["cb_phonelist"] and cgi.params["phonelist"]