spot check
noun [ C ]
ukusan examination of a company, activity, or process to make sure that everything is being done according to the rules, without warning the people involved that this is going to happen:
a spot check of/on sth Since last year's disclosure of illegal practices, there has been an increase in spot checks of local offices.
make/do/carry out a spot check Lenders can make spot checks to see whether you have defaulted on past loans.