hi, the most concrete way to determine this is to do a search for the word in the iambic sections of aeschylus, sophocles or euripides and see if the syllable sits in (out of the 12 positions) positions 3, 7 or 11 (which only accept light syllables) in which case the vowel is short, or in any of the even positions (2, 4, 6, 8, 10 or 12) which only accept heavy syllables, provided that it's in a single unresolved syllable (i.e. there aren't 2 short syllables in that position) and is in an open syllable (i.e. the syllable isn't made heavy by having a short vowel but closed by consonants), in which case the vowel is long (that's a bit complicated i realise).
so in your case, see aeschylus persians 405: θήκας τε προγόνων: νῦν ὑπὲρ πάντων ἀ
ἀ is in position 11, and so it's short.
you can't trust any of the vowels in positions 1, 5 or 9 because these are ancipites which accept light or heavy syllables.
for more background see pgs 3-4 of my article here on writing iambics:http://www.aoidoi.org/articles/meter/WritingIambics.pdf
and then see how i have determined the vowel length in aeschylus' agamemnon in the red lines on the left on pgs 58 to 69 of this iambic document here:http://mhninaeide.webs.com/GrkIambicComp-23-Apr-06.pdf
in that doc i followed the procedure i mentioned at the beginning - to determine the vowel length in the ancipites, i found the vowel in another line in a non-anceps position.