spot check

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An inspection or investigation that is carried out at random or limited to a few instances.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A cursory inspection or examination or the inspection or examination of a sample of something.
• verb idiomatic To inspect or examine briefly, or by sampling.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a check on work performance or product quality made at random times without warning