Finding out if upgrading to a newer macOS version will cause issues? I'm going to be upgrading my Macbook Pro (mid-2012 model) from its original OS X Lion 10.7 installation to a newer version of macOS. It looks like Apple will allow me to upgrade from Lion directly to OS X El Capitan 10.11 and macOS Sierra 10.12, and from there all the way to macOS Mojave 10.14.

I have heard that upgrading to a newer OS can cause performance issues, especially on older hardware. How can I find out about these issues, so I can determine the best version of macOS to upgrade to?
The biggest consideration is the software you run, not just in terms of compatibility with macOS generally, but also in terms of identifying if they're 32-bit or 64-bit apps (which will be a crucial consideration moving forward if macOS Catalina interests you, since it will be compatible with your MacBook Pro hardware).

As a starting point you can use the RoaringApps website to check software compatibility. You can search by specific application if you have key software you need to work with. You can also actually download a Roaring Apps app for macOS, but I've always found the Application Compatibility Table (see below) on their website to meet my needs.

My advice would be to check your software against the Application Compatibility Table for compatibility with El Capitan. If all good, then upgrade to that first and use the system for long enough to be satisfied everything is okay, and long enough to have a full working backup of your El Capitan setup.

Once you've done that, check the Application Compatibility Table again for compatibility with macOS Sierra etc before upgrading again.

To identify which applications you have installed are still 32-bit, you can follow one of the answers at: How can I find out which Mac apps are 32-bit?