spot check 
noun
A test made without warning on a randomly selected subject: I made a spot check on each engine...
- A spot check of people out shopping on a nice spring day offers insight into what must be a growing phenomenon: the company store.
- In our spot check of more than 30 different routes, we found savings that ranged from as little as $5 to as much as $75.
- The Scarborough scandal was uncovered when a spot check by the Audit Commission of 41 hospitals in England and Wales revealed that all but three issued false figures.
verb
(spot-check) [with object] Subject to a spot check: releases into the water of radionuclides are usually spot-checked rather than continuously monitored...
- A Cancap technologist tests the water daily; a military preventive medicine technician spot-checks it at all stages, and weekly samples are sent to an independent laboratory in the Persian Gulf.
- No public accountant spot-checks his findings by ringing, say, 20 alleged Liberal voters.
- If this general officer was so willing to pencil-whip his personal weapons qualification record, how willing and able do you think he was in spot-checking his troops?