The quickest way to check the Python version is to run this command in the command line:
python --version
Another, and quicker way (note the capital v):
python -V
Both commands will work on all popular systems: Windows, Linux, and Mac.

More information
You can add another capital v, if you want more information, like the Python version, release date.
It will also display whether you use the 32-bit or 64-bit versions.
python -VV

Python versions
There are two major Python versions: Python 2 and Python 3 that are not fully compatible.
If both versions are installed on the machine you can check for Python 2 using this command:
python --version
or
python2 --version
If you want to check python3 version, use this command:
python3 --version
Check Python version inside the code
To check the version of Python interpreter inside the script you have to import the sys module first, and then display the Python version.
import sys
print(sys.version)
The print function will display this result:
3.7.2 (tags/v3.7.2:9a3ffc0492, Dec 23 2018, 23:09:28) [MSC v.1916 64 bit (AMD64)]
You can also display the system version info:
print(sys.version_info)
Result:
sys.version_info(major=3, minor=7, micro=2, releaselevel='final', serial=0)