Never Ever Gamble

The situation: Your sister is doing a coin flip and you're supposed to guess which will be the facing side up. Heads, my dear, or tails. According to probability laws, it's supposed to be 50-50 (assuming that sides are equal and that your sister is not cheating).
However, it is my belief that 50-50 does not begin to describe the odds I have because really, who is to say I am not getting all ten or so times wrong out of a potential thousand? Perhaps, you know, it's just the odds against me for now...