Ok, this is not about false diphthong /ou̯/(ου), 'cause it split with long /uː/ (but anybody know a certain time of this spliting? In Wiki this describes simple "at early times") and disappeared from phonological system at all. So there is not problem with the distinction.
But, as described at Wiki, diphthong /ei̯/ still remains in the phonology, and we have a some kinda disposition - there is still true and false diphthong ει.
So, but how to distinct the one from the other!!? I have found a little description that there are so rules for the distinction, but nothing else (by condition of my asking here):
diphthongs distinct at writings by placing some accent marks to the next syllable.
But what about exlamation '¡εί!' - '¡hey!' !? It must be pronounced as /ei̯/, and not as /eː/, or not!?
Wiktionary still have this issue (or not). So maybe who know more complex explanation how to separate the one from the other?
P.S. Why here isn't AG section at the Stack~!?