Teaching assistants (TAs), students who assist the faculty, are widely used in computer science (CS) courses. Previous studies have, however, shown that TAs could be poorly prepared and need training. Particularly, an interview study has shown that one of the areas where the TAs experience uncertainty is when assessing students’ oral presentations of their lab assignments. Based on that result and by interviewing course coordinators, we have developed and offered training workshops about assessment in CS. We invited our TAs in the introductory CS courses to participate on a voluntary basis. By distributing pre-workshop surveys at the beginning of each semester, and post-workshop surveys at the end of the semesters, to both workshop attendees (50) and non-attendees (44), we studied how the TAs conducted the assessments and what impact the training workshop had on their self-reported practice. Both surveys had 11 identical statements that the TAs were asked to rate on a 7-point Likert scale. We also conducted interviews with four workshop attendees and three non-attendees. The results showed a significant difference between the two groups in the post-workshop survey: workshop attendees disagreed more with the statement “I try to assess students' understanding rather than the program”, which is more in line with the instructions given. In addition, when comparing pre- and post-workshop answers, the workshop attendees stated that they were less inclined to ask for help, experienced that the lab instructions were not detailed enough, and were more inclined to ask questions that convinced them that the students had written the program themselves. In the control group, no significant differences between pre- and post-tests were found.