How to tell when "ui" is a diphthong or in hiatus?

Question

In Spanish, there is no orthographic distinction between the diphthongs [wi ~ uj] and the vowel hiatus [ui]. Is there a way to predict how "ui" is pronounced in a word?

Possibly via phonetic environment (i.e. the preceding consonants)?

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It seems to me that, for example, the sequence "cui" is generally a diphthong (e.g. cuídate), and that verbs in "-uir" are generally in hiatus (e.g. construir). Can such generalisations be made? The RAE seems to imply the realisation of "ui" are heavily dialect, sociolect, and idiolect dependent:

Por otra parte, algunas de estas combinaciones vocálicas (las formadas por... dos vocales cerradas diferentes) pueden, en una misma palabra, fluctuar en su pronunciación entre el hiato y el diptongo, dependiendo de diversos factores, como el mayor o menor esmero en la pronunciación, el origen geográfico o social del hablante, etc.; así ocurre, por ejemplo, en gratuito, que puede pronunciarse con diptongo (gra - tui - to) o con hiato (gra - tu - i - to)...

• DPD: Diptongo 2.

Answer 1

Regarding the hiatus [u.i], all verb infinitives ending in -uir have it, and it's inherited by all their inflections and derivations where it may apply: substituir, substituido; recluir, recluido; fluir, fluido, fluidez; huir, huida, etc. I think this is so because we are used to verbs getting stress in the last vowel of the infinitive; we only recognize three verbal classes (-ar, -er, -ir). It might also have to do with the fact that in many of the finite forms of the verb, the vowel /u/ remains clearly part of the root while the /i/ is changed to /ʝ/. This reinforces the idea that the /u/ must be left alone.

Now, it shouldn't be strange to find speakers that turn the hiatus into a diphthong in some cases, like when the stress moves elsewhere. I pronounce huidizo with a hiatus but fluidificar (if that were a word) I think I would pronounce with a diphthong. (There are not many examples to choose and try.)

Outside the cases above, most instances of ui are pronounced as the rising diphthong [wi].

The falling diphthong [uj] is usually orthographically marked as uy, and the only examples of it that come to mind are muy and cuy, both monosyllables. The proper place name Ibicuy is also [ibi'kuj]. I don't think I've ever heard unstressed [uj], although the difference might be hard to perceive. Notably, the variant cuis (for cuy) is pronounced [kwis].