The next step in the research process is to see if anyone has done any work on your line, so you will need to do a survey of online family trees.
Online family trees are notoriously unreliable. They must be used with great caution.
Rules of Thumb
1. The more sources a family tree has, the more likely it is to be accurate.
2. When you're constructing your own family trees, shoot for at least three sources for every person you add.
3. What sources do you add for each person? All of them. I will cover some basic rules of evidence later on about how to decide if a source matches the person, and what sources mean.
Doing the Survey
Choose a deceased ancestor in the line you will be researching. Preferably it should be someone who was alive during the 1920-1940 US Censuses. Search for this person (and if he or she doesn't show up, look for a parent or grandparent if you know their names) in the following databases:
Ancestry (LDS members have free access; others can subscribe or use an institutional copy at Family History Centers or public libraries)
Case Study: The Lintons
I will start with Samuel Linton (1827-1916). It looks like his Philadelphia family is well represented in FamilySearch Family Tree; they have between 2 and 24 sources each, so it's worth looking through each entry and double-checking the vital records and sourcing those who are only partly represented.
In Ancestry, there are dozens if not hundreds of family trees, with between 1 and 17 sources, so there will be a good database of information to sort through.
In MyHeritage, I set up my LDS account and began a family tree. It was a fairly seamless process, but I'm not sure I want to maintain multiple family trees. However, I see eight matches for Samuel Linton with complete family information and dozens with partial information. A spot check suggests other people are using the tree in a similar manner and I might not find extensive new information through these trees, but it's worth a try. MyHeritage has the largest membership of any of the genealogy sites, so along with Ancestry it's the most likely place I'll find Linton cousins if any of them still live in Philadelphia.
RootsWeb WorldConnect is a free online family tree service maintained by Ancestry. Much of the information is dated, but about a decade ago it was one of the default places to store family trees, so it's worth a look. There seem to be two family trees available. One has sources, so I will look at that.
It seems to have the same information as all the other sites do, including many misspelled place names, so I won't bother using it, but it WorldConnect occasionally contains valuable compiled databases by expert genealogists, so it's worth checking.
Check these sites for your own family line. Do you see cousins working on your ancestors, or are you doing original research?
And do remember that online family trees are only as good as the work that went into them, so check how thoroughly sourced they are and if the information looks at all credible before copying any of it over. It may be worth contacting the owners of the family trees if you don't know who they are and what connection they have to your family.
Philadelphia Genealogy: index to all articles in this series