Apple's diagnostics would capture some logs but you'd need Apple to tell you which apps are stuck or chewing through the battery / CPU / energy.
I would focus on checking your last backup and erase things selectively.
If you want to erase slowly:
- reset all settings and then power cycle - that's quick but usually doesn't fix this
- erase all content and settings - then connect to power and do light web apps Safari - don't sign into iCloud, don't restore the backup, don't install any (or many) apps.
Run with the stock OS that was causing the problem without your data or apps to convince yourself that it was your programs and data. If you still see the power drain, then restore the device from iTunes or Apple Configurator or have a technician do it.
- If it's hardware - none of these will fix it.
- If you had a corrupt iOS - the restore from USB will fix it
- If you had programs crashing and data continually trying to update (software bugs or such) - the erase all content and settings will fix it.
- If a trivial setting got stuck and you needed a minor fix - the reset settings will fix it.
- The last thing it could be is simply a process that takes a long time to finish. Photos face recognition or upgrading from an old iCloud storage format to a new one can take three or more days to sync if you have 100 GB of photos with many people in them and your photos database has a lot of things to process. It might not even be something you need to fix and can connect to power and WiFi wait this out.