If you look at the status bar that shows your health and mana levels, you will see 5 little squares just underneath them. These are the square that show how many concentration spells you have used. If they are lit up (light blue on my screen), then they are in use. Spells that use concentration slots are usually buffs that you have cast. While I can't remember off the top of my head which spells they are, I know that Magi Shielding is one of them. If you watch as you buff yourself the next time, you will notice which ones fill one of these squares.
If you wish to charm something, unfortunately you will need 3 open concentration slots as that is what Charm uses. If you don't have 3 open concentration slots, then you will get the error message that you don't have enough concentration to cast Charm. This means that you will have to assess which maintained spells you really want to keep up if they take up a concentration slot.
I hope that this helps to answer your question.