According to the documentation.
If you are running on the TensorFlow or CNTK backends, your code will automatically run on GPU if any available GPU is detected.
You can check what all devices are used by tensorflow by -
from tensorflow.python.client import device_lib
print(device_lib.list_local_devices())
Also as suggested in this answer
import tensorflow as tf
sess = tf.Session(config=tf.ConfigProto(log_device_placement=True))
This will print whether your tensorflow is using a CPU or a GPU backend. If you are running this command in jupyter notebook, check out the console from where you have launched the notebook.
If you are sceptic whether you have installed the tensorflow gpu version or not. You can install the gpu version via pip.
pip install tensorflow-gpu