Unfortunately there's no rule available, for proper names. Accents on toponyms, like in graeca verba, sine lege vagantur.
Just to name a few in my surroundings:
- Sambrusón (it should be Sanbrusón, but that's another matter)
- Grùmolo (delle Abbadesse)
Some names have suffixes that help in guessing the right accent, but, as you see from the last three, it's not so easy.
I once saw a list of towns in Calabria: placing the accents was just a wild guess. Some toponyms that are frequently mispronounced are
- San Lazzaro di Sàvena (not Savéna)
- Friùli (not Frìuli)
- Nùoro (not Nuòro)
Family names have the same problem; those derived from job names are usually guessable at, but the region of origin can influence the pronunciation. A family name coming from Veneto and ending in -on or -er should have the accent on the last syllable: Benettón and Venièr; but Mìlan can be rightfully pronounced with the accent on the first syllable, depending on its origin, while Bressàn (it means “from Brescia”) can never be Brèssan.