A very reasonable question, OldAg54. It is the easist thing in the world to falsify the flying hours in a logbook -but very difficult indeed to get away with it for a sustained period. Aviation is an extremely small world and if the hours claimed in a logbook don't look right then people start asking questions and checking up. This is taken very seriously by airlines and the authorities. It is very rare but does happen.
If a pilot is shown to have made false claims, it is not just a matter of ending his flying career but a prosecution by the CAA and prison sentence.