You can pay with a previous payout ticket, your member card, or cash. Often (if not always), the previous payout ticket will go in the same slot the cash can be inserted. Most machines take $5, $10, $20, $50, $100 dollar bills, though some also take $1, too. These areas will be lit up and unmissable.
To find the payout percentage on a slot machine, you can look for information provided by the casino or the game manufacturer. In many jurisdictions, casinos are required to display the payout percentage for each slot machine.