If you stole your step moms credit card and buy something will she find out through credit card bills what you bought?

Absolutely she will ! Assuming she checks her statements each month to see what she spent (and where) - she'll instantly see where her card was used ! Hope you're ready to face the consequences when you're caught out ! AND - I'm not just talking about you getting a 'telling off' either - you should be doing jail time !)