No, I don’t think “open CUDA library” is enough to tell, because different nodes of the graph may be on different devices.
When using tensorflow2:
print("Num GPUs Available: ", len(tf.config.list_physical_devices('GPU')))
For tensorflow1, to find out which device is used, you can enable log device placement like this:
sess = tf.Session(config=tf.ConfigProto(log_device_placement=True))
Check your console for this type of output.