Sometimes it not easy to determine, which pin on in IC package is pin 1. On square packages such as QFP and QFN variants, often there are several circular marks, of which only one is the pin 1 marker, the rest are traces of the ejectors which push the IC body out of the mold. On some packages - SSOP and the smallest BGA/WLCSP), sometimes there is no marker for pin 1 at all.

Luckily, ST is very consistent in marking their chips: if the chip is positioned so that the marking text is readable, pin 1 is at the left bottom corner.

One notable exception from this "rule" is the UFBGA169 package (STM32xxxAxHx) with ball A1 at the left upper corner.

Drawings of packages confirming this information used to be in the errata couple of years ago, and in the last few years ST placed them into the datasheets. However, the very latest datasheets don't contain these drawings anymore, and the reference for them is now TN1433.