What is a test called that involves indicating whether a line has the same slope as a previous image?
The task sounds like a match-to-sample task which is indeed used in working memory studies. In this test participants are presented with a sample stimulus and have to judge whether or not following stimuli resemble this particular stimulus. Depending on the time span between the presentation of the sample stimulus and the following stimuli this task is sometimes also called a delayed match-to-sample task.