It really depends upon the casino platform that you are using to play the casino games. Some of the online casinos will track your wagers on a free bonus, and you can go into the casino cashier to find out how much you've already wagered and how much more needs to be wagered before you can withdraw your winnings. It's always best to wager more than the bare minimum, to avoid being labeled as a bonus abuser.
Many online casinos do not keep track of your wagers. You'll need to figure out the total amount that needs to be wagered for your casino bonus, and then try to keep a running tally in your head on the amount of wagers that you've already made. It's not always easy to keep track of your wagers, but you should have a general idea of when you've met the wagering requirements and it's safe to cashout your winnings.
Some of the casinos will void your winnings if you attempt to cash-out before you've met the wagering requirements, even if you believe that you've already placed enough bets. If you are unsure of the number of wagers that you've made within the casino, then you should contact customer support and ask them if you've met the wagering requirements. They will be able to tell you how many wagers that you've made.