Elsewhere on the forum has examples of reading the Vdd of the Simblee and will report back the voltage on Vdd.
If you are using batteries without the boost chip, you can test and see how low Vdd gets to test to replace batteries.
But if using the boost chip, Vdd will always report 3.3v until the batteries die.
I have simply done a analogRead on a pin to check voltage levels of the batteries while using the boost chip feature. Using the boost chip allows for LED usage while straight battery the LEDS will dim.