If it is in right, you won't feel the tampon at all and the string will be hanging out of your vagina. If you can feel the tampon or you feel pressure in there you may not have put it in completely right. No big deal! Just take it out, get a new one and start over. Every woman's vagina is different and some women have to put the tampon applicator in straight up while others have to angle it back just a little bit. Be sure to push the plunger all the way in so the tampon goes up high enough and then you won't feel it at all.
You'll know the tampon is in right if the applicator comes out easily and comfortably, if you don't feel the tampon once the applicator is removed, and if there is no leaking. If you are new to tampons, relax. You get better at it every time you use them. My daughter recently started using tampons and it took her several months of "practice" before she was comfortable enough to use them regularly.
I know my tampon is in right if I can't feel it. It's, of course, a little easier for me to gauge based on the fact that I've been using tampons for more than 4 years. If you're just switching to tampons or have just started your period, it'd be a good idea to learn how to insert a tampon first. You can ask your mother, a close girlfriend, or read our guide on how to properly insert a tampon. Once you know how to insert a tampon, the rest is easy-peasy (excuse my corny saying). It's in right if you can't feel it. If you need to, try again. Thankfully, they typically give you more than 10 or 20 tampons in one box.