I am a 23 year old mom, and have not been out of "teenage-hood" that long, and let me tell you what I remember... also, note that this is strictly my experience.
You will NEVER, EVER in a MILLION YEARS earn their trust or respect by spying and snooping. Talk to them. Ask questions. This will be far more effective. They won't like it much, but at least you're being honest. Also, it opens the door for them to talk to you if you talk to them first.
My mother was very open with us. She asked questions about where we were going, where we'd been, what was up with our lives and the like... I didn't always answer honestly, but I did probably 95% of the time, and when the stuff hit the fan (and many times before it had the chance to) she was the first person I talked to, because I knew she cared enough to ask. I was a pretty good kid. Never any major trouble (I credit much of this to being able to be open with my mom, even when I was in trouble) but I always had someone to go to when I needed it.
For example... I've never smoked pot, abused a medication, and didn't drink until I was 21. Why? Because I didn't have a reason. I didn't have a reason to rebel, I had a mom who cared about my choice in friends (she always invited my friends over for dinner and for outings), and she asked questions. Loving, interested-in-my-life questions. I always knew, when I had a choice when I was with my friends... after I had made that choice... there would be questions.
My parents were seperated, and my dad was the opposite. He snooped, dug around our rooms, etc. It made me not want to tell him anything. I knew his game, and I played it right back.
If you can help it, be their friend, not their competitor. You'll get miles and miles further.
Chances are, that snooping or not, they'll get themselves into trouble. It's if you're there when they have questions or problems that really counts.
Hope this helps. Probably different that what you'll here elsewhere, but it is my experience. :) Good luck with your step-boys!