How to tell if the arduino pro mini is 5V (16mhz) or 3.3V (8MHZ)

if it is not checked on the back side, the best 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

mine reads 80 u (something that looks like the letter U) so it is 3v3, here is a photo

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