Just because your mother’s file does not have sources, does not prove whether or not she was a careful researcher. She may have been carefully adding to family over a period of years. It might be wise to take what you were given and attempting to document the family’s generational links one by one as you received the file. As you begin to prove her work you may decide it is a good starting point for you, or you may want to start anew.
If you decide to use her file as your starting point, start verifying with yourself, and add source information on parents, grandparents, great-grandparents one at a time adding sources as you go generation by generation. These should be in the form of vital records, bible records, wills, land records, old family letters and diaries, census records, obituaries, historical books, tombstone inscriptions, published family genealogies, etc. Information from shared family trees online such as from FamilySearch, Ancestry, MyHeritage, and collections of information such as family genealogies, etc. needs to be verified by comparing to the most reliable records possible which are recorded as near as possible to the event in time.
Adding large amounts of information from any large database is a problem because you cannot attempt to document vast amounts of information without looking at individual persons on a one to one basis.
Ancestry does have many historical records online as well as shared databases. When they added census documents a few years back, this was really helpful to me, because I could check these in the middle of the night from home rather than taking road trips to libraries and National Archives to view the census films. There are of course several companies that offer records online, so one does not have to rely on Ancestry or other favored company to find original sources. FamilySearch has online many original source documents that one would be unable to travel in a lifetime to view.
I guess the most important point to remember is that verification is best done one person at a time, gradually expanding each family group one generation at a time, back in time. At least if a family has a relative who has a good start on a family, it is easier to verify one generation at a time if one has some seemingly reliable information, that to start completely from scratch. Question everything from shared family trees. There is both very good information and very unreliable information out there. It is not always easy to tell the difference.