Typically, if an app, movie/show is not free, there will be indicator that lets you know that a subscription is required or there is a charge for the rental. Some will have free trials. Something like Starz would require a subscription. You can navigate to the free section on Roku to see what is free. You can also do a search for free content as well. The Roku channel itself is also free.
This is also dependent on what subscriptions you have. For example, I still use Spectrum TV and pay for a monthly cable subscription, which includes certain things as part of the service, such as ESPN plus, Disney, etc...