Generally, If the Oscillator is 16Mz, the voltage will be 5v
Why Arduino pro mini
Arduino pro mini board has almost all features of Arduino Uno except the programming interface. After programming, the programming interface components become redundant and not useful in most of the cases. In Any system, the rule of thump holds good "More the complexity, more the problems". Arduino pro mini provides the minimal set of components
Arduino mini comes in 5v/16Mz or 3.3v/8Mz forms.
How to identify Voltage and frequency.
Very straight forward method is to measure it straight. Connect a supply voltage upto 12 to "RAW" input compared to ground ("GND") and measure the voltage available at VCC compared to ground.
Another approach is to read labels on voltage regulator and resonator (oscillator ).
Below image shows the components.
You may need a lens to read this.
The voltage regulator IC has a label "S8PE". this corresponds to 5v
The label on the Oscillator reads like "Ag". Anything starting with "A" corresponds to 16Mz.
Those Oscillator starting with "8" is an indication of 8Mz