if it is not checked on the back side, the simplest and fastest way to do this is to look at the resonator, (Clock) – usually a Crystal oscillator, but I don’t see why it can’t be a ceramic resonator, if it has something with an 8 in it, then it is probably an 8MHZ resonator, otherwise it is probably 16, where 8 comes with the 3.3V and 16 with the 5V (Scroll down to see an example)
Another way is to connect the RAW to, let’s say 12V or even 5V, and measure the voltage at the VCC pin, that would also tell you, but surely, this involves more than just looking at the board, you need to connect wires and a power supply !
mine reads 80 u (something that looks like the letter U) so it is 3v3, here is a photo
If your resonator does not read anything with an 8 on it, (My other one reads A.P), chances are it is 5 Volts
The CP2102 breakout board shown in the photo below has a selector between 3.3V and 5V.
In my case, i simply connected it the way you see it here to a USB port with 3.3V, and what do you know, it works, I have a flashing red light on the Arduino, a constant green light, and it looks like it is read to take code, the CP2102 board has a constant red light.
if you want to check that it is actually working fine, simply upload the sketch blink no delay, and then alter it a bit so that it does a double blink then wait 2 seconds, now your code is working, there you have it.
From the device manager, I can see that there is a device that looks like my adapter, namely this one
Silicon Labs CP210x USB to UART Bridge (Com3)
That thing above tells me that it has been designated the communication port number 3, which i will need in the Arduino IDE
So now i have installed and am running the Arduino IDE, selected a 3.3V Arduino board from the list and selected Com3, now i should be ready to upload a sketch, let us make a sketch that double flashes an LED every 2 seconds. here is some code to do that