there are multiple patch are available.
that we install in our site
i just want a script to find out which patch are install in my magento site.
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Like Krishna said, you can check applied.patches.list.
Also there is a good checker which helps for normal users without server details or dont know how to check.
You will get what you need and even more :)
You can find the applied patches
applied.patches.list in the app/etc/ directory.
When we apply patches this files are generated automatically.
Next to that the patch is nothing more than a diff file so you can see what it should change when you open it and check in the targeted file if it is indeed changed.
Additionally to other people's answers Philip Jackson developped a small free module to be able to check the applied patches from the backend. It's available here: https://github.com/philwinkle/Philwinkle_AppliedPatches