First, tap on Bets on the bottom banner of the display screen. Once you have reached the Past Bets page, you can see all your previously placed wagers and sort through them by selecting one of the time range buttons highlighted below. At the moment, you will only be able to go back a month from the app and website.
The plus and minus signs are shorthand to tell you what your payout will be if you win your bet. If you see a minus sign next to your bet, your payout will be less than 100% of your initial investment if you win the bet. If you see a plus sign, your payout will be more than 100% of your initial investment.