* Shine on, you, crazy diamond !

Strange, but in Sonoff Basic red led does not show relay status, as on TH10/TH16. But there is a pin labeled LED on connector for external RF module. And it lights up red led.

According to https://www.itead.cc/wiki/File:Sonoff-Schematic.pdf, the relay is controlled via Q1 gate:

Q1 pin 1 – dotted, so I have soldered wire between it and 3d pin of RF connector.

Now, when the relay is ON – red led will shine.

Quite simple ;)