The back sensor had an indent too, so you could easily feel it. Hold your phone in your hand. Where does your index finger sit? If it is on the back of the phone, that's where the sensor was. It was very natural.
In that case, I'm grabbing it with my thumb, index finger and middle finger, and since the phone is seated upside down, my thumb reaches the home button before I'm even holding it in my palm.
On phones with a fingerprint sensor on the back, I first have to get a full grip, and then I can reach for the sensor.
(But it's probably somewhat hand/pocket-specific...)