You'll see it in regular dictionaries. Lewis and Short will often say whether something is "rare" or "very frequent" and sometimes will even indicate the period in which the frequency changes. It does it not only for words, but also definitions.
For example, let's take the word scelero:
I.perf., ātum, 1, v. a. id., to pollute, defile, contaminate, desecrate (in the verb. finit. rare, and only poet.; “syn.: temero, polluo)
You'll see that not only is the finite form of the verb rare, it is only found in poetry. You wouldn't use it (for whatever reason) if you're construction and oration or writing a history. Additionally, it provides synonyms so you know which words to choose instead.
Or with sanguineus:
I. Lit., of blood, consisting of blood, bloody, blood- (class.; a favorite word of the Aug. poets)...
II. Transf., blood-colored, blood-red (poet. and in postAug. prose): “jubae
It chiefly relates to blood (including some metaphorical uses, like in bloodthirsty) in classical literature, but in poetry and in post-Augustan (so "Silver Age" and later) prose, it also denotes the color red.
A related word, sanguinarius, shows additional notes:
I.of or belonging to blood, blood-,
I. Lit.: herba, an herb that stanches blood, the Gr. πολύγονον, Col. 7, 5, 19; “also called sanguinaria alone,” Plin. 27, 12, 91, § 113, and sanguinalis herba, Col. 6, 12 fin.; Cels. 2, 33; 3, 22 fin.: latus sanguinare, covered with blood, Vulg. Ecclus. 42, 5.—
II. Trop., blood-thirsty, bloody, sanguinary (rare but class.): “juventus,” Cic. Att. 2, 7, 3: “Claudius (with saevus),” Suet. Claud. 34: “bella (with cruenta),” Just. 29, 3, 3: “sententiae,” Plin. Ep. 4, 22, 6: “illud responsum,” Plin. 19, 8, 53, § 169.
So there you'll see two definitions contrasted. Sanguinarius usually denotes something physically related to blood. Metaphorical usages is relatively rare, but since Cicero uses it once in this way, it is "permitted" if you want to compose Latin in good, Classical style.
Lewis and Short is inexact, and of course much work has been done on frequency since, such as automatic corpora scanning. A good resource in my opinion is the Dickinson College Commentaries website, which has a section on "core vocabulary," and it includes the rank assigned to each word.
You'll have to be careful, though, since some websites which show frequency don't group inflected words, so alii will sit alongside alia in the list. (See this one for example.)