How can you tell when the SMC is reset successfully?

I don't believe there is an indicator as such if it's successful.

In Ye Olde Days, when Macs had hard drives, removable batteries and a charge indicator on the left side, that would light up and sometimes the pilot lamp on the MagSafe plug would blink.

But nowadays your only bet is that if the problem that caused you to do the reset disappears, then you know it worked...

You can actually tell, using the MagSafe charger. If you have your charger plugged in, and the SMC reset is successful, then the LED indicator on the charger will momentarily change from status (Amber to green or visa versa).