The Mental Measurements Yearbook is the primary database for finding reviews of major published assessments. Reviews will include validity and reliability as well as background on how the instrument was developed.
PsycINFO, and ERIC will provide you with additional information about how an instrument is being used and some articles comparing it to other instruments. These databases can also help you identify a test on a particular topic.
Below is a list of the major test and/or assessment publishers.Their web sites often have valuable information about an assessment instrument.