bbPress.org

You could temporarily turn off pretty permalinks and you’ll see their id in the url.

You can also use this mini-plugin to show their user id in the profile

add_filter( 'get_profile_info_keys','bb_member_id_in_profile',255);

function bb_member_id_in_profile($keys) {

global $self;

if (empty($self)==true && isset($_GET['tab'])==false && bb_get_location()=="profile-page") {

(array) $keys=array_merge(array_slice((array) $keys, 0 , 1), array('ID' => array(0, __('Member #'))), array_slice((array) $keys,  1));

}

return (array) $keys;

}