Looking for people without parents is different than looking for "islands" of people not tied into your main tree.
Looking for people without parents will be dominated by a list of spouses of people in your tree (or top-of-line ancestors of spouses) - which I don't think is probably what you are looking for.
FTM has a report, called the "Extended Family Chart" which will include all of the island of people related to a person, but you still have the problem that you would need to run that report for each person in your database to find "unrelated islands" or "Orphaned Islands" of people
There have been programs over the years that will identify all unrelated people to a base person in a gedcom. (That is, it will give the opposite of the Extended Family Chart - everybody NOT related to a person.) I believe one used to be called gedanalyze or some such title. This was maybe 10 years ago or so. If downloads are still available, the program should still work, as gedcom hasn't changed much in the past 10 years.