Yes – but you should always check on the website.
Most online slots should give you information of payout percentages for each slot before you commit your bet to your spin – this will give you an idea of how much you are set to make from a successful spin. The payout percentage shows how much of your winnings will go to you as your payout, with the remainder returning to profit the casino.
Free slots generally don’t display their payout percentages – because the money involved isn’t real, there would be no point.