How do you find the truth when genealogical records don't jive?

Many people even 100 years ago didn't read and write, so their name was written how it sounded by the various people such as parish clerks, official officers, enumerators etc and people were not precious about spellings as they didn't know, so you will get different spellings, which is why you get at least three records and cross reference to get nearest to the true information as is possible...one of my ancestors in roughly the same time frame has four different spelling to his name, it seems he also decided to add a middle name later in life, he certainly wasn't christened with it, didn't use it on marriage or in the census, but his Will, all his farm survey plans and on private letters he uses it and in looking at 6 census years there is a 20 year discrepancy in his age...thankfully it was towards the last 4 census and I know it is the correct person as he lived in the same house and the house is still family owned..............cross reference everything you can get your hands on...........................