How does one tell if a a greek vowel is long or short from the dictionary?
Some dictionaries use the proper symbols “against” the dubious cases, others nothing at all.
One could start by learning the quantity of the word endings first, then observe the accents and the relevant rules (they’re not so many).
What remains is the dubious quantity in word-roots, which also gets easier when you observe related words that you already know.
If you need to know the quantity for reading poetry aloud, you can spare much effort by just ignoring the θέσει μακρά (long by position), because these count as long anyway.
Hope this helps
I assume you mean ⍺, ⍳ and υ which can be both long or short. Most dictionaries don’t show this, that’s true but Diogenes does which you will find here: http://www.aoidoi.org/diogenes/
This is of course not the best place to post this kind of question (Learning Greek section is best) - hence the delay in replying but hopefully it may still be of use despite the delay in replying.