I've inherited several Magento projects, and one is back on Magento 1.4. I'd like to upgrade this project to 1.9.x but I don't know if previous developers have made any changes to Magento core as the project has had a history of developers with questionable practices.
I doubt there's many options except upgrading and testing in a non-production environment but wanted to check to see if there's a way to see what has been changed, if anything.
Are you using any VCS, if so then I suggest that you get a copy of the same magento version you currently have (1.4) and copy all the core files directly to your existing doc root, this way your VCS (git, svn) should be able to tell you which files are different and also will tell you which lines where modified.
You can get any version of magento from here:
The AOE Guys made a good plugin to magerun where you can find core hacks I reckon you should check it out: https://github.com/AOEpeople/mpmd/blob/master/README.md#command-mpmdcorehacks
On the other hand, if you're using a version control system, you can simply do a diff between a fresh Magento install and your files.
Literally copying over the files in version control works, but it can get messy.
Another option, if you're on Windows, is to use WinMerge. It lets you select two separate directory trees and compares them recursively. Text files (i.e. all Magento source files) show a nice diff with changes. So, I would grab a vanilla copy of Magento matching the site's version, then run WinMerge on directories like app/code/core, lib, and so on. Alternatively, you can just compare the site's root directory, and right-click and "exclude" diffs you don't care about.
