Note: BST36 slide guides are pictured, but the concepts and specifications are the same.
New. Notice the 0.010" deep depressions in the bottom corners concentric to the bore (circled in green), as well as the four circular depressions.
Contrast this with the following two pictures of worn guides. The first wear example shows a guide at 8,300 miles (13,358 km). Wear has exceeded the depth of the depressions at the lower end of the slide excursion/concentric to the bore (normally found in the areas circled in red). Note the plastic dust in the circular depressions. Maximum measured wear is ~0.020" (0.51 mm). Emulsion tube wear ensues rapidly when the slide guide has worn beyond ~0.010" (0.25 mm). That is, once the depressions within the areas circumscribed by the colored circles have worn away.
The third photo shows a guide at 36,000 miles (57,936 km). The depressions at the lower end of the slide excursion/concentric to the bore (once again normally found in the areas circled in red) as well as the four circular depressions are worn completely away.