I have a basket of LCDs collected over the years. I'm trying to build some circuits now but want to restrict my power supply to 3.3 volts meaning an HD44780U chip for the LCD and avoid having to include a 5 volt supply for the HD44780S unit. Unfortunately, I don't see any identifying markings on my LCDs so I'm not sure what they are. Is there a way of telling ? My circuits aren't working so far so I can't use trial and error.
If you can't see the markings, you can't really be sure if the controller chip even is an actual Hitachi HD44780 or just one of the many compatible clones from multiple manufacturers.
And being a HD44780U only guarantees that the controller chip itself works at 3.3 supply. It does not mean that the LCD glass can operate at 3.3V. Some LCD modules might have an onboard voltage converter to get enough drive voltage for the display when used at 3.3V. The ones meant for 5V operation certainly don't have any onboard voltage converters.
However, if it can be assumed that they really have HD44780 controllers, the difference between S and U models can be determined from the font. The chips have a couple of standard fonts and they can be ordered with custom fonts.
The HD44780U has more font ROM so it defines more glyphs. UA00 comes with a "Japanese standard font" and UA02 comes with a "European standard font". So careful observation of different character codes may reveal if it is a U or S model.