When you attempt to download an app that costs money, you will not be charged if you already bought the app. After you press buy and enter your iTunes password, it will say "You have already purchased this item. To download it again for free, select OK."
The problem is that I deleted hundreds of apps from my phone, some of which were paid for. Later on I might find an app I like but notice that it costs money. It might be an app I had already paid for, or an app which I got when it was free.
How can I know if I tap the buy button if I will be charged for the app or not?
In other words, how can I know if I owned a previous version of an app before?