ATtiny85 (Sparfun Clone/Trinket), detect/read operation voltage
There are a few solutions. 


If you want to save te state of your toggle button you can save it in the eeprom of your attiny85
You can use an ADC pin to check your VCC against the internal 1.1V reference
alternatively you can read the 1.1ref aganist Avcc. https://digistump.com/board/index.php?topic=1104.0