I have a 16 year old daughter, and yes I would have a look at her stuff. I have a close relationship with her and she has already approached me and told me that she has smoked pot (weed) She told me that she tried it a few times but decided that she didn't like the feeling.

But back to looking through your kids things, yes it's ok, if you have a good reason for thinking that your kids are doing the wrong things. Also you have to think really hard about how you would react if you do find things you don't like. Because once you've found something, you have to deal with it in a positive way where your kids don't shut you out even more. 

So devaloping a open relationship with them and having trust going both ways is always the best way to know what your kids are doing. I have done this by honesty becoming friends with her, I talk with her, and have got to know her friends very well. I let her friends stay in the weekends and spend alot of time with her. In return she trusts me and has told me about her fears of things such as sex and drugs.