One clue that they were naturalized would be a notation in the Census. There is a column that often contains a simple Na - sometimes the adjacent column gives a year. That would be further indication that they were naturalized. If it is missing, it doesn't mean it didn't happen later. Again, for a woman, it is often simply a tag-along of her husband's naturalization status.
Some older folks may have applied for naturalization once Social Security was enacted.