I'm taking a course on Middle-High German this semester, and the first week or so, we focused on pronunciation.
The main example I have is from Hilkert Weddige: "Die Diphthonge <ei, ou, ie, uo> und die Umlaute <öu/eu/öi> (für /öu/) und <üe> sind auch als Zweilaute zu sprechen, und zwar wie im Bairischen mit fallender Betonung, z.B. líe-be, gúo-te, br´üe-der. Nur das <iu> ist als im Mhd. ein langer Monophthong [y:], z.B. triuwe." (Mittelhochdeutsch, Eine Einführung, p. 13)
My translation: "The diphthongs <ei, ou, ie, uo> and the umlauts <öu/eu/öi> (for /öu/) and <üe> are to be spoken as two sounds, and, like in Bavarian, with falling stress, for example líe-be, gúo-te, br´üe-der. Only /iu/ is spoken as the long monophthong [y:] (or [ü:]?), like triuwe.
In modern German, for example, /ie/ is pronounced as a long [i:], so how do we know that those were pronounced as diphthongs, while /iu/ is a monophthong?
/r/linguistics might be able to help you more, however one common method of figuring out pronunciation is by analyzing poetry and song lyrics. From there its not too difficult to make educated guess about pronunciation.