My understanding is if there's flavoring, it will appear somewhere on the label, and if you see it on the label, it could be problematic. (Occasionally the flavoring itself is just herbs and not a kosher problem, but there's no way for a consumer to tell what flavoring is in his "flavored" beer.)
Here's the US Government saying that if you add anything other than the "normal beer ingredients and processes" (and if you're not sure what that means, ask them), you have to notify them with some extra paperwork; that's probably related to this issue.
(a) For what fermented products must a formula be filed? You must file a formula for approval by TTB if you intend to produce:
(1) Any fermented product that will be treated by any processing, filtration, or other method of manufacture that is not generally recognized as a traditional process in the production of a fermented beverage designated as “beer,” “ale,” “porter,” “stout,” “lager,” or “malt liquor.” For purposes of this paragraph:
(i) Removal of any volume of water from beer, filtration of beer to substantially change the color, flavor, or character, separation of beer into different components, reverse osmosis, concentration of beer, and ion exchange treatments are examples of non-traditional processes for which you must file a formula.
(ii) Pasteurization, filtration prior to bottling, filtration in lieu of pasteurization, centrifuging for clarity, lagering, carbonation, and blending are examples of traditional processes for which you do not need to file a formula.
(iii) If you have questions about whether or not use of a particular process not listed in this section requires the filing of a formula, you may request a determination from TTB in accordance with paragraph (f) of this section.
(2) Any fermented product to which flavors or other nonbeverage ingredients (other than hop extract) containing alcohol will be added.
(3) Subject to paragraph (f) of this section, any fermented product to which coloring or natural or artificial flavors will be added.
(4) Subject to paragraph (f) of this section, any fermented product to which fruit, fruit juice, fruit concentrate, herbs, spices, honey, maple syrup, or other food materials will be added.
(5) Saké, including flavored saké and sparkling saké.