How can I know if my NN TensorFlow model is overfitted or not?
Short answer, no it does not look like you are overfitting. Your validation (you call this test) accuracy seems to follow your training accuracy quite closely. 

Easiest way to check for yourself is to plot the training and validation accuracy over each iteration, then look at how the curves behave. What you want to see is a test curve that closely follows your validation curve. 

Overfitting can come in too ways, your validation curve being quite a bit lower than your training curve and/or a validation curve that is decreasing when the training curve is increasing.

Both these behaviors can be seen in this picture:



Image taken from Stanford's CS231n

Edit: As gunes points out you seem to have a programming error that should be addressed before any checks for overfitting.