Absolutely you can. And it's been done. Teen's are sneaky and dishonest that way, it's the nature of the beast. That's why I've been known to reach over and just grab it from her at random times and inspect it. The vast majority of the times there's nothing to see. But there have been things discovered that we've needed to discuss and times when her "privilege" has been revoked because of it. The alternative Facebook page (we suspected it anyway) was a good one. That one earned her a 9 weeks of school with no phone. And I've seen reference to being the "cool" parent. I have no interest in being the "cool" parent, just the mature, responsible parent who puts the safety and well being of my children above all else. She may resent it now but she'll thank me one day. I know I resented the hell out of my parents for many things but as I've matured and become a parent myself I've come to realize why they did the things they did and how I benefited from it.
Strict parents create sneaky kids, don't forget that.