Test the pin #1 of some 74 TTL and find the connector pin getting in touch with a multimeter. You will find the 5v one.
Don't have the ST shematics with me at this moment, but if you take a look at them, you will find the chips that are using the 12v (example: for rs-232). Then do the same thing at test what is connected to the other pin of the PSU connector.
It is the best solution for me if you don't have any pint-out informations...