As you attack each syllable (either with pen or tongue) ask yourself these questions, in this order or in an order adapted to your own needs:
1) Do I immediately recognize it as a long or short syllable, because it has:
A) a dipthong
B) two consonants that make position after the vowel
C) a vowel I remember to be long by nature, either in the stem (e.g. in many perfects)
or in the ending:
(noun/adj.: nom pl in i or es, gen sing in i or plural in orum or arum, dat or abl pl in is, acc pl in os or as or es or is, abl sing in o or a
verbs: long o in 1st sing. [usually], stem vowel a, e, or i in 1st, 2nd, 3rd conjugation, tense vowel a or e in imperfect, etc.)
D) a vowel I remember to be short by nature, not followed by two consonants that make position (e.g. abl in e, gen in is, neut pl in a)
2) (May be used prior to #1.) Is it a freebie? Does its place in the verse make it obvious, because
A) It's the first foot of a dactyl or spondee, and so long3) For still uncertain cases:
B) It comes after a long and a short, and so has to be another short
C) I'm in a freebie zone, such the last two feet of the hexameter, which pretty much have to be "dum de de dum dum." (Some advise starting here and working back.)
D) I'm in the second syllable of a dactyl or spondee, and I can see out of the corner of my eye that the next syllable is clearly long or short (see #1 above): if the next syllable is long, this must be long, to finish off the spondee; if its short, this must be too, to make the dactyl. (You'll never have "long-short-long" as in the name "Scipio")
A) Look more seriously at the next syllable, to see if its being long or short may help. Do this to help with ambiguous mute + liquids.4) If you're stuck, try this "troubleshooting"
B) Check your commentary for a note on any metrical oddity
C) Go to the freebie at the end of the verse, and work backwards.
D) Look up the length of the stem vowel in a dictionary, or check (and resolve to learn) the length of the ending in a grammar or intro book (or #1C above).
A) Have I missed a syllable?5) If none of this works, make a note to ask about it later. Don't lose any sleep. Put some time into meter, but don't let it get you down, don't become obsessed with it, and remember that the goal is to enjoy the poetry more, not less. It would be great if could say each line either aloud or to yourself as you read/translate.
B) Have I missed or misread a dipthong, or mute plus liquid (poets have some flexibility here)?
C) Have I missed an elision? Or have I elided something that the poet wants to leave unelided (in "hiatus"-- he can do this).