It probably goes without saying that to assess students’ ability to write code, you could ask them to write code. Additionally, consider providing students code that they need to modify to change the behavior of the code. This can provide students practice reading code and identifying the important features that determine each behavior.
Example: "Let's practice all of the skills at the same time!"
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-assessing-programming