Teenagers think about sex and seek to have it. If she's experimenting with others her age, then the best you can do is have the kind of relationship with her where she knows she can come to you with any questions or concerns she has.
If you think she's being groomed by older men or something sinister, then yes that warrants an investigation.
But if there's no actual danger to be feared, then being respectful of your teenager and trusting her is the best course.
Please don't hack her phone/facebook/instragram etc she will find out and be FURIOUS and it's a massive breech of privacy. Does she have a boyfriend? Any guys on the scene? It's definatly time ti have a conversation with her a out protecting herself CONDOMS always and other contraception. Respect for herself and others... and hope she listens.... you really can't investigate
I have a dd who is 13 and I regularly check her iPad after she has gone to bed (I don't let her keep it in her room overnight and make her plug it in to charge in the kitchen, same with her mobile). She doesn't know that I check (although she might suspect it, she never deletes her chats with her friends) but I do. If I found something I wasn't happy about I wouldn't let on that I'd found out (unless it was something really awful) but I'd try to start conversations about whatever it was in the hope that she might open up to me.
If I suspected she was having sex at a young age I would be very strict about who's house she was going to and dropping her off and picking her up myself etc. Yes she could be really sneaky about it but I would really make every effort to make it very difficult for her.
We do have a very open relationship with each other (despite me sounding like an ogre I'm sure) and we talk about sex and relationships etc she knows all about safe sex. I had sex for the first time aged 18 and I would be really disappointed if my dd had sex aged 14 or so.
Have a conversation about contraception. It's what my mum did. She said that she couldn't stop me having sex, but if I was going to do it, she would at least be able to rest a bit easier knowing I was safe from pregnancy and infections.
Isn't that illegal ? A reminder of that might not be a bad idea ! And obviously the other party is breaking the law too !!! I'd be giving an extremely short sharp lecture on the law /age of consent if I were you ! Good luck !