Custom taxonomy if statement

I’m guessing you want to display that information on a single post or a post type?

is_tax() conditional tag is used to determine if the archive page of a particular taxonomy/term is displayed. (documented in codex)

What you’re probably looking for is has_term() function (documented in codex).

So in your case the full condition would be:

<?php if ( has_term( 'part-time', 'job-time' ) ) { ?>
    Yes
<?php } elseif ( has_term( 'full-time', 'job-time' ) ) { ?>
    No
<?php } ?>

NOTE: That works withing the default loop (checks the current post), if you want to check term of some other post, you could pass a post ID as a third parameter.