I usually vote up a question if it meets any of these conditions:
- I can answer it with the information given and I don't see any reason why it should be closed
- Someone else has answered it well or I know someone else will be able to answer it and I don't see any reason why it should be closed
- It seems to be a useful/interesting question and includes enough information to answer it in theory and is on topic (I upvote even if voting to close as dupe)
I usually vote down a question if:
- it is a rant
- I think it is unanswerable with the information given after allowing time for OP to respond to comments
- I don't think it could help anyone else
I find questions to vote on:
- by watching the feed, reading the front page
- doing reviews
- following links posted in comments, in answers, and in the sidebar to linked/related questions
- using the stats page in mod tools
- doing cleanup (I use a data query, my old comments, my review history)