For genealogists and family historians, a particularly vexing research problem is the inconsistency with which family names were spelled over the span of many years. These spelling inconsistencies can make it difficult to track down and verify family relationships and/or confirm individuals related to events. Just why does the name keep changing in the first place, and how do you know you are researching the right person when the surname seems to change so frequently?
In answer to the initial half of the question, first, middle and last names were all subject to frequent misspelling, depending on who was doing the writing when your ancestors' documents were originally penned. The reasons for these errors varied, but included: lack of concern for accuracy by the writer or subject person; grammatical inability of the writer; difficulty understanding the given name (perhaps the writer was of a different nationality, and did not recognize the subject persons’ spoken words - a common error with census enumerators); simple copycat errors (the writer saw, and repeated previous misspelled references).
To the second part of the question of how one knows they are on the right track researching an individual, one way to consistently verify the identity of the subject person is through a process I call surname "landmarking". Landmarking fixes a known name to a known time and a known place, and perhaps a known occupation, thereby establishing a consistent trail to confirm the individuals' or family's identity. To be successful with this technique, I recommend researching records chronologically, year-by-year, to better evaluate the information as it is discovered.
There are numerous public records suitable for the landmarking process, but the ones referred to herein are the State and Federal Censuses, which were conducted at 5-year intervals between one another, City Directories and Tax Rolls, which were published annually, and Property Indentures.
