The Bal Shem Tov says if you want to know whether you are really serving G-d, it is simple:  

If my heart is filled with joy each time I put on tefillin, and each time I do something good my heart is filled with joy, then I am serving G-d.  

If I am not on that level, then I am just doing mechanical things.  

Even that is very holy, and I’ll be rewarded in Heaven for doing it, but it is heartbreaking.

~ R’ Shlomo Carlebach zt”l