OctoPrint will tell you what Python version it is running under at the same place it also show you its own version, in the lower left corner of the web interface:
The systeminfo bundle will also contain this information, and if you upload a bundle to bundleviewer.octoprint.org it will show you a warning if you are still on Python 2.
If OctoPrint says it's running under Python 3, you are good and not going to be left behind with OctoPrint's dropping of Python 2 support in version 1.8.0.
Don't be alarmed if you run
python --version on the command line, e.g. on OctoPi (confused about OctoPi vs OctoPrint? Read this), and it still shows you 2.7. By default, the Debian distro that OctoPi is based on still has the default
python command still pointed to a Python 2 installation. However, OctoPrint has it's own so called virtual environment created with Python 3, and that's what's important. So as long as the Python version as reported by OctoPrint says Python 3, you are good!