Flask is one of the popular web frameworks in python. To check the version of the flask package installed in an environment, you can use one of the following methods.

Using python interpreter

import flask
flask.__version__

Using command line

flask --version

Using pip (if flask is installed using pip or easy_install command)

In Linux OS

pip freeze | grep flask

In Windows OS

pip freeze | findstr "Flask"   

I hope this tip is useful. Feel free to comment in case of any issues.