Latin texts leave the distinction between high monophthongs [i, u, ī, ū] and glides [j, w] unspecified. This has lead some to suggest that the glides are allophones of the monophthongs. For instance, Steriade (1984) implies that the syllabicity of [+high, +vocalic] segments in Latin is largely predictable. Steriade points out two contexts where high vocoids are (almost) always glides: initially before a vowel (# __ V) and intervocalically (V __ V). In these two contexts, the only complications I am aware of arise from competition between generalizations. For instance, in ūua [uː.wa] ‘grape’ and ūuidus [uː.wi.dus] ‘damp’,  intervocalic glide formation appears to bleed word-initial glide formation. (Or it could be the case that ū is ineligible for glide formation by virtue of its length.) And the behavior of two adjacent high vocoids flanked by vowels is somewhat idiosyncratic: compare naevus [naj.wus] ‘birthmark’ and saeuiō [saj.wi.oː] ‘I am furious’, where (by hypothesis) /ViuV/ surfaces as [j.w], to dēuius [deː.wi.us] ‘devious’ and pauiō [pa.wi.oː] ‘I beat’, where (by hypothesis) /VuiV/ surfaces as [.wi] but never as *[w.j]. And so on.

However, Cser (2012) claims that syllabicity of high vocoids is not at all predictable after a consonant and before a vowel, i.e., in the context C __ V. Here we usually observe [w] when the preceding consonant is coda [j, l, r], as in the aforementioned naevus or silua [sil.wa] ‘forest’. Cser contrasts this latter form with belua ‘wild beast’, which is trisyllabic rather than bisyllabic. However, it is not clear this is a good near-minimal pair. The word was clearly not pronounced as [be.lu.a] because the first syllable scans heavy. In the following hexameter verse, the word comprises the fifth foot, a dactyl:

et centumgeminus Briareus, ac belua Lernae (Verg., Aen. 6.287)

Lewis & Short and the Oxford Latin Dictionary both give this word as bēlua [beː.lu.a]. However, it seems much more likely that the word is in fact bellua [bel.lu.a], as it was sometimes written. (Note also that tautomorphemic geminate ll is robustly attested in Latin.) In this case we would expect glide formation to be blocked because the [lw] complex onset is totally unattested, just as Cser predicts from general principles of sonority sequencing. Thus the above verse is:

[et.ken|tũː.ge.mi|nus.bri.a|re.u.sak|bel.lu.a|ler.naj]

As Cser notes, many of the remaining near-minimal pairs occur at morphological boundaries⁠—and thus look to someone with my theoretical commitments as evidence for the phonological cycle—or relate to the complex onsets qu [kw] and su [sw], which might be treated as contour segments underlyingly. But much work will be needed to show that these apparent exceptions follow from the grammar of Latin.

References

Cser, András. 2012. The role of sonority in the phonology of Latin. In Parker, Steve (ed.), The sonority controversy, pages 39-64. Berlin: Mouton de Gruyter.
Steriade, Donca. 1984. Glides and vowels in Romanian. In Proceedings of the Berkeley Lingusitics Society, pages 47-64.