Upvote:2
Refer to the eurail site itself for all that information - see here for information on reservation on regional trains; in short: there's very little exceptions to the rule that regional trains don't require reservation. Here is a list (by country) of where reservation is needed or optional and what the cost would be.
There's also a feature to avoid trains that require reservation in any search you make on the site - see here for explanation.
Especially in France required reservations are very common - that's why eurail has a page on this very issue and gives alternative routes for some of the most popular routes involving France.