Alright, alright, we can hear you laughing already. A girl who often buys new clothes? That just means she's still breathing, not that she's necessarily cheating. We know that no woman would ever turn down an opportunity to expand her wardrobe, and your lady is likely to be just the same. But sometimes, just sometimes, it's not a good sign.
The key thing to look out for is whether she's actually wearing those clothes around you or not. Even if it's as she's getting ready for a night out with the girls, she should still be showing off her new rags proudly. On the other hand, if you're seeing a bunch of receipts and shopping bags but none of the merchandise, she's saving her best looks for someone else. And she doesn't want you to know.