Train strikes in Italy can be a major inconvenience for travelers, especially if you're caught off-guard. These strikes are scheduled in advance, however, and the dates and times are announced and posted. This means you can find out about them, in some cases, before you even leave home.
The best resources for learning about strikes is in Italian. It's the Commisione di Garanzia Sciopero's site, which is regularly updated with information on strikes around the country, both having to do with trains and other sectors. If you have a little understanding of Italian, or know someone who does, this can be an invaluable resource.
An English site that's good, if not as detailed as the Italian site, is Easy Travel Report. Click on the link for "Italian Strikes" to see Italy information.
Once you're on the ground, you can sometimes find out about strikes from the Italian news. Just listen for the word "sciopero," which means "strike" in Italian.
If you don't find out about a strike until you arrive at the train station, be prepared for a long wait at the ticket counter as officials try to accommodate every traveler who's been inconvenienced by the strike.
Note that on days when strikes are held, Trenitalia guarantees minimum transport services. These services are a direct result of an agreement between Trenitalia and the trade unions.