How can I test if a field has been assigned to a channel? I've got a generic template that can display entries from different channels. How in the template can I test if a field has been has been assigned to any given channel?
Twig has a "is defined" test that you can apply to a variable to see if it exists:

{% if entry.possibleField is defined %}
    Yup!
{% else %}
    Nope!
{% endif %}


And here is more information on using is defined.