Strikes are announced in advance, the unions that organise them are required to by law. Sometimes they're cancelled, sometimes they're rescheduled, but recently they've always been on a Friday, or starting Thursday night - convenient for the weekend eh? So for the people of Italy who rely on public transport to get around it's a matter of keeping an eye on the news and on the transport companies' sites to see what's planned.
The thing is with the strikes is that even if they're announced for 24 hours or any other period, public transport doesn't come to a complete and absolute stop for that amount of time. There's always what's called the fasce garantite, the periods during the day when service is guaranteed. In Milan today, as it usually happens when there's a strike, service is guaranteed for urban transport until 8.45am and then again from 3-6pm. There are also certain trains that are guaranteed in case of strikes, which are indicated on the timetables in train stations. Outside these times you'll still see various mezzi around, the strikes have a varying level of success, especially as there are many different unions involved - some drivers won't be members of the unions on strike. Even so, here the metro is always the worst hit. Today outside of the guaranteed period of service, the underground is completely closed. So when there's a strike it's not necessarily impossible to get around using public transport, but very difficult.
For people who have a limited knowledge of Italian it can be difficult finding out about these strikes. On the Ministry of Transport site's there's information about planned strikes, but only in Italian. If you're in Milan you can check the ATM site, which is in English here (even though I can't actually seem to find any news in the English section about today's strike..). After not finding anything in the English language news from the Italian news agencies, I'd recommend running Italian news about strikes through an online translator. The translation may not be accurate, but at least you'll be informed to a certain level.
If you're unfortunate enough to experience a transport strike, here are some words you may hear used:
Lo sciopero: strike
I mezzi: transport
- mezzi urbani: urban transport
- mezzi interurbani: interurban transport
- fascia garantita: the guaranteed period/s of the day when service will be running as usual
La circolazione: circulation
Il disagio (singular) / i disagi (plural): disruption
superficie: surface (when referring to non-underground services)
Essere in agitazione: to be on strike