See this page for the original source:
http://alex.leonard.ie/2011/06/30/wordpress-check-if-post-is-in-custom-taxonomy/

As of WP 3.2 – there’s no core conditional function for checking whether a post belongs to a taxonomy term. A bit of searching around found this WordPress Support forum post by Justin Tadlock explaining how do check against a specific taxonomy. I’ve expanded this function a little bit so that it can be used generically.

/**
* Conditional function to check if post belongs to term in a custom taxonomy.
*
* @param    tax        string                taxonomy to which the term belons
* @param    term    int|string|array    attributes of shortcode
* @param    _post    int                    post id to be checked
* @return             BOOL                True if term is matched, false otherwise
*/
function pa_in_taxonomy($tax, $term, $_post = NULL) {

// if neither tax nor term are specified, return false
if ( !$tax || !$term ) { return FALSE; }

// if post parameter is given, get it, otherwise use $GLOBALS to get post
if ( $_post ) {
$_post = get_post( $_post );
} else {
$_post =& $GLOBALS['post'];
}

// if no post return false
if ( !$_post ) { return FALSE; }

// check whether post matches term belongin to tax
$return = is_object_in_term( $_post->ID, $tax, $term );

// if error returned, then return false
if ( is_wp_error( $return ) ) { return FALSE; }

return $return;
}

Let’s say you’ve created a custom post type called “Films” which has a custom taxonomy of “Genres” and you want to know if the current post is in the “western” genre, you would do as follows:

if (pa_in_taxonomy('genres', 'western')) {
echo "Western films are great";
} else {
// do something else
}

Note I’m using the tax/term slug, but the is_object_in_term function in use will accept term id, slug, or array of either, so you could do:

if (pa_in_taxonomy('genres', array('western', 'drama', 'comedy'))) {}
// or
if (pa_in_taxonomy('genres', array(1, 2, 3)) {}