In the UK we have this thing called income tax. With most people it is 'Pay As You Earn', hence, when starting a job, something has to happen to calculate how much tax is to be paid. I would like to know if it is possible to infer one's previous salary from this information. Also, are there any 'golden windows' where this can not be a possible problem, e.g. by starting a job in a new tax year (April), will it be possible for a lie about previous salary grade to go unnoticed?