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For example, could I use chicken broth (that does not contain any actual chicken meat) to prepare a meal on Friday?  What about chicken bouillon?

No, and no.   In December 2015, there was a closely related discussion elsewhere on CathInfo, as the Vigil of the Nativity (Christmas Eve) was nearly upon us.

I provided a Web address for a more recent, albeit more, um, partisan, statement on traditional fast & abstinence:

    <web.archive.org/web/20150429100201/http://www.stmichaels.org/fasting-laws.shtml>.

It's more focused on when the various rules applied, but some excerpts are directly applicable herein (after omitting the obvious ones):

2. [...] On days of complete abstinence, meat and soup or gravy made from meat may not be used at all.

In a source that seemed reasonably authoritative (but for which I can't find a citation now), I read that there's what seems to be a concession to--or recognition of--differences in culinary cultures: Use of only the rendered fat, drippings, or grease (from the same animals whose broth, soup, or gravy is not allowed) are allowed only when used as a substitute liquid cooking medium distinct from those of other cultures, notably butter (e.g.: French) or oil (e.g.: Italian).  I doubt that, e.g., many servings of eggs & hash-browns were fried in extra-virgin olive oil out on the American frontier.

5. Eating between meals is not permitted, but liquids [other than those in item 2], including milk and fruit juices, are allowed.

The source I'm excerpting is apparently squeamish about pointing out in item 5 that beer, wine, and other alcoholic beverages are, in fact, also allowed.  Recall monks in mediæval Europe, brewing beer or fermenting wine or distilling brandy as products whose sale can contribute to the upkeep of the monastery.  Then, if you not reading this when it could impact your appetite, ponder the health risks from plain water that courses downstream to the monastery thro' a secular settlement that more-or-less surrounds its walls.  And recall that it would be several--or many--centuries before the role of microscopic organisms in causing human diseases would begin to be understood.  Be that as it may, for whatever is allowed to be eaten or imbibed, rules of fasting do not allow the excessive quantities that indicate gluttony or cause drunkenness.

I was really asking for situations when you are eating out or eating processed food, and you're not really sure what's in it.

I had a few surprises related to that concern, in late December:
&#183 Canned cream-based clam chowder, which listed "chicken broth" among its ingredients.
&#183 Shrimp-&-grits recipes on line, some from up-scale sources, for which all (maybe except 1) of nearly a half dozen used bacon grease, a few then crumbling the crisply cooked bacon over the top.  No olive oil for those folks, who were predominately from the U.S. South.  I'll leave it to those here with formal theological training to explain whether or not a "don't ask" attitude is a morally defensible when sitting down to a breakfast or brunch menu in Charleston (S.C.)

• on a Friday.

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Note *: "[Re:] Fasting and Abstinence this week - Christmas Eve but NOT Christmas Day", in "Catholic Living in the Modern World" forum (f=8). Dec 23, 2015, 3:00 pm. <www.cathinfo.com/catholic.php?a=topic&t=39269#p3>.

Note #: Many early settlers of Charleston were the French heretics known as Huguenots, so I wouldn't be surprised if catering to "papist" traditions were not a (collective) cultural priority there.

Thanks for the input, everyone.  It's interesting, I was at the grocery store last night and was thinking about what to get for lunch tomorrow.  I was going to buy canned vegetable soup, but noticed that it contained beef stock.  I then saw a soup called "vegetarian vegetable soup," which was made from vegetable stock instead of beef stock.  I had never really paid that close attention to what actually contains meat and what does not until now.