You can check whether there are free seats on trains directly on the operator’s website. For internal trains in Italy, that would be trenitalia.com. Please note that Italy is not a very “Interrail-friendly” country to travel in. Especially if you have a non-continuous Interrail pass (of the type “5 days in one month” etc) and you are going to travel in other countries, too. It might well be that some trips inside Italy are cheaper to buy as normal tickets than what the price of your Interrail pass divided by the number of days it is valid is. Plus, for the high-speed trains there is a 10€ surcharge if you use Interrail that you need to take into account, too. Then it might be better to “save” the travel days in other countries.
On the other hand, it is annoying to buy a “10 days in two months” type of pass and then notice when your trip is about to end that you have unused travel days left. Optimising what tickets to use for what trip is really a hard problem.