Ask three questions in order: can this hand win the high half, can it win the low half, and do the cards on the table help or hurt either plan? A hand that answers yes to the first two is a scoop hand, and scoop hands are worth more than anything else because they play for the whole pot while most opponents play for half of it.
High potential
High potential is what the hand would be worth in Seven Card Stud: three of a kind, a big pair, three suited cards, three connected cards. In Stud 8 that value gets a discount, because whenever a low qualifies, the best high hand only collects half. The discount is smallest when nobody is going low, which is why a pure high hand is best played when the door cards around you are high too.
Low potential
Low potential means three different ranks of 8 or lower, with the ace counting as the lowest card. The lower the three ranks, the better, and three cards that can also make a wheel (A-2-3-4-5) carry the most value because the wheel is a straight for high as well. Low potential on its own buys you half the pot at most, and when two players make the same low that half is split again. Low hand rankings explains how 8-or-better lows are compared.
What the exposed cards say
The door cards tell you two things before you put in any action: whether the cards you need are still live, and who is competing for which half. Three low door cards around you mean more competition for the low and fewer low cards left in the deck. Three high door cards mean your low draw may be the only one at the table. Reading exposed cards in stud covers the counting routine; this page applies it to the starting-hand decision.