how can I know if my tumblr is 100% anonymous?

[Summer]

because i have a private tumblr and someone i know added it and how can i know if it was by chance or if it was because they know it was me im freaking out.

[Miaaaaa87]

I'm actually confused because I don't know if you meant someone added you on a private tumblr or you want to make sure your tumblr has no indication of who you are...

If you mean private tumblr like this:
http://letsfightlikekingkongs.tumblr.com/

A private blog is locked with password and if someone tries to add, tumblr will say "Sorry. This feature isn't available on protected blogs." ...So if someone added you on that still, it's not private. You can check by logging off and going to the private tumblr.

If you just meant "anonymous" as in no indication who you are... As long as you don't have your name, pic, real life details, twitter/fb/myspace link etc on your tumblr.... No one will know. It's chance.