It's strange: the other answers are written in English, and I'm a native English speaker, but I don't understand! what you're all saying about not voting on topics you don't understand.
(Jump to the bottom if you want the secret to finding good Questions and Answers to vote for.)
In my experience, most bad questions that come along aren't "oh, that's a silly thing to do, why would you ask about that?" What we mostly get is crap that's badly asked, unclear, off-topic, incomplete, home use, whatever.
It's easy to find stuff to downvote:
When I'm reviewing close votes, almost everything that I VTC also gets a downvote if it doesn't already have one.
I've written before about finding crap to downvote so that the auto-delete will get it. The vast majority of questions that are a few days old but have no votes and no answers deserve a downvote.
As Iain has pointed out more than once, the majority of questions from people with 1 or 101 rep are crap.
And when it comes to judging good questions, I really don't understand comments like @MichaelHampton's:
I won't vote on a great question about Active Directory if I don't understand it.
Really? My knowledge of AD is limited - the last version I actively used was Win2003 - but I can still recognize a good AD question. I just had a look at the newest [Active-Directory] questions and even though I couldn't answer most of them, I can tell which ones are good and which ones aren't.
Again, the bad ones usually aren't just bad, they're steaming piles of crap - easy to identify. OTOH, almost every question tagged AD (on the first two pages) that's clear and understandable is good enough to upvote. There are very few that are bad because the person is asking about doing something insane or dangerous.
And hey, look, most of the good questions are asked by people with more than 1 or 101 reputation.
Which leads me to something that I thought was obvious, but maybe not:
If you want to find good Questions and Answers, look for Answers that high-rep users have written.
Click on the user page for anyone with high rep and look at their answers. You'll usually find that their answers are good and so are the questions they're answering. These users don't really need any more rep, but voting for their answers and for the questions they're answering is to help make the good content stand out.
Summary
Reading my answer over again, it seems like I approach voting differently than most people - I go looking specifically for either good stuff to vote up or bad stuff to vote down. I don't just let the questions wash over me (I used to - I used to read every excerpt on the /Questions page and most of the actual questions).