You look it up on the app or website when you decide you might like to do it. Amtrak has multiple yield management buckets and inventory is allocated to them. Both Flex and Value step up through the buckets as the inventory allocated to each bucket sells out and the next one up opens.
Airlines are not particularly forthcoming, either, about their yield management practices on fares when the same restrictions apply. Since all Amtrak fares are either Flex or Value, the same restrictions apply to all fare buckets. It appears to me under current practices that they share the same inventory buckets, with an upcharge for Flex. I think that is a change from their former practice when there were much greater fare differences, a highly restricted fare type called Saver could sell out and Flex appeared pretty much fixed at high bucket. Now it looks to me like both Flex and Value are available now at all bucket price points with a relatively small price difference between them and Saver was eliminated.