How to identify Arduino Mini Pro 5v vs 3.3v
Consideration (go down for the actual solution)

If you have a lot of different China-Arduino-clones, the easiest way will be to just test it. The cloned Arduinos use a lot of different voltage regulators and often you won't even be able to find datasheets for them on Google.

Someone here provided an Arduino sketch which helps you identify the frequency with which the Atmel is running and the comments say "it is the safest method". But why? It is complicated (you need to connect the USB-TTL-converter to each of the different Arduinos and sometimes they have different pinouts, so it's error-prone - you could easily fry your Arduino if you connect the wires to the wrong pins)...



Solution

I think the safest way is just connecting a voltage in the range of 6V-12V to the RAW-pin and measuring the (regulated) output of the VCC-pin. 

There is no reason why this would kill your Arduino (in case your Arduino isn't broken - which would be good to know anyway; and of course you shouldn't short anything).

I've done a short sketch of how you should connect your Arduino Pro Mini:



RAW &lt;---> 6V-12V
VCC &lt;---> multimeter (5V = 5V Arduino, 3.3V = 3.3V Arduino)
One GND pins to the GND of the power source and one GND pins to the GND of the multimeter (you can take any of the Arduino pins labeled with GND)


Don't worry if your Arduino looks a little bit different, just look for the pin labels. For instance, one of my 5V Arduino Pro Minis looks like this, but also has RAW-, VCC- and GND-labels (just one of the GND pins is at a different position, make sure you check your Arduino pin-labels):




Alternative

You don't want to use the method above for some reasons? Then you need sharp eyes and some luck that the following list includes you components (some of these are collected from the other posts, some of them from my Arduinos):


3.3V:


Voltage regulators: KB33, S20K, F34V, L0RA, L0RB
Oscillators: 80e., 80'0

5V:


Voltage regulators: KB50, L05, L0UA, L0UB
Oscillators: R160JAC6s, 16.000-30, A1, A'N, A'a



(no limit or warranty for these details...)



More Suggestions

Some of the Arduino Pro Minis come with MHz and V markings on the back. In my case, none of these were checked. Make sure you check them as your new Arduinos arrive, so you won't have any problems identifying them later!