A vinyl test pressing is just that, a test to be evaluated by the band, label,  or re-issuer prior to the print run being started.  If the Test Pressing is deemed acceptable, the retail run of the product begins.

Test pressings have a white or plain label, and they are in a plain white jacket.  There is no identifying marking on a test pressing, save for the serial number in the dead wax of the LP.

Ideally, a test pressing will sound exactly like its retail issue counterpart- there is very little, if any, audio difference between the test pressing and the regular issue.  The reason for their higher price is their considerable collectability, not for superior sound.

As with most things, there is a exception to the above stated rule.  When dealing with a vintage test pressing (i.e. pre 1987 or so) you may actually hear a sonic advantage.  This has to do with the large size of print runs during vinyl’s heyday.  Print runs were so large during the golden era of LP's that copies done early in the print run often sounded better than copies done late in the print run, due to degradation of the stamper over the course of the run.  Obviously, test pressings were the first pressings from those stampers, so a slight audio advantage may be obtained in some cases.

In the modern era of LP pressing, however, the print run sizes are so comparatively small that no degradation occurs during the printing process, and the test pressing sounds virtually identical to the retail product.