The resonator is a silver component located besides the AVR. However it's so tiny I can't see the figures on top.
I had another suggestion: measuring the voltage between VCC output and GND. And indeed the board is a 3.3 version. But in between I have checked my orders and discovered I had indeed ordered 3.3V boards, not only 5V boards. I couldn't check visually because the board's bottom is covered with Velcro to tie the board to cloth. So I guess there's no error from the maker but from me only. Duh.