For many people, what they do defines their character. A professional, such as a doctor, lawyer, teacher, and so on may exude confidence while someone who's a waitress, stock person, or menial laborer may not. The former often belongs to professional organizations that would have records of their members. The latter most likely may have been active in their church or synagogue.
The level of education that a person achieves is often directly related to their occupation. While it's not possible to work in many professions without further education, it is possible to be a farmer, construction or factory worker, and such with the bare minimum. Up to World War II, many families, especially those of immigrants, concentrated their efforts at further education to the male siblings. Girls learned from their mothers how to keep house and raise a family, which is what the majority ended up doing. Sometimes, only the oldest son got to go to college, while the others worked to put him through.
An ancestor's occupation often determined where they lived. Occupations in the 18th,19th, and early 20th centuries were tied to the Earth's geology. Suppose one of your ancestors was a farmer and settled in central rural Pennsylvania in 1810. But you aren't sure just where he settled. Farmers in those days settled where there was fertile bottom land, notably near a river. This land allowed for easier plowing–they had to plow with a horse or mule, pushing a hand-held plow behind them–and the soil was rich in nutrients. The river also provided a means of transportation to the nearest large settlement. And while the Monongehela and Allegheny Rivers flow in the western part of the state, the only major river in central Pennsylvania is the Susquehanna. By studying a topographic map, you should be able to tell where the country is flat or semi-flat near that river. Most likely this will be the region to explore.
You can also find out about an ancestor's occupation by process of elimination. If you know that your ancestor was a female and arrived in the country when she was older, spoke no English, or had an infirmity, the odds are that she probably was a housewife or cleaned or took in laundry. She obviously wouldn't have been in the military, but she could have been killed in a skirmish, especially during the Civil War, so that rules out finding any information in military records.
The reverse might also work for you. If you know that your ancestor lived in Bedford, Massachusetts in the 1840s, for example, you can almost make a bet that he would have been involved in the whaling industry as that town was the biggest whaling port on the East Coast at that time. You'll have to look for other clues in his background to find out exactly what he did. He may have built or repaired boats, he could also have been a fisherman, a cooper (a person who built and repaired the barrels for holding whale oil), a sailmaker, or a member of the crew of a whaling ship. And if he spoke English well, he may have even been the captain of a whaling ship. If he were a Quaker, he may have own the whaling ship. And while at this point everything you discover is conjecture, each piece of information fits into a larger puzzle.
If your ancestor was an unmarried female, and lived before 1930, chances are good she worked as a store or office clerk, waitress, secretary, cook, or librarian. If she was an immigrant and didn't speak much English, she may have worked as a cleaning lady, dishwasher, laundress, kitchen helper, or in a sewing factory.
And while all these are generalities, they give you a point from which to search for long-lost relatives.