This depends on how you installed TensorFlow.
Pip installation
Run:
python -c 'import tensorflow as tf; print(tf.__version__)' # for Python 2
python3 -c 'import tensorflow as tf; print(tf.__version__)' # for Python 3
Note that python is symlinked to /usr/bin/python3 in some Linux distributions, so use pythoninstead of python3 in these cases.
pip list | grep tensorflow for Python 2 or pip3 list | grep tensorflow for Python 3 will also show the version of Tensorflow installed.
Virtualenv installation
Run:
python -c 'import tensorflow as tf; print(tf.__version__)' # for both Python 2 and Python 3
pip list | grep tensorflow will also show the version of Tensorflow installed.
Examples
For example, I have installed TensorFlow 0.12.1 in a virtualenv for Python 3. So, I get:

and installed TensorFlow from source for Python 2 (not in a virtualenv).

Referenced: