You can easily find your missing assignments by looking at the Progress Reports page. Once you choose a course to look at, you can look at the "Grade" column for anything that says "Missing".
You can also use the "Missing Only" filter to make it easy to view just your missing assignments. This can be useful if you have a long list of assessments and don't want to scan through each one. Click "Yes" to view only your missing assignments. Note that Schoolrunner will hide all of your other assessments in an "Other" category – this just represents all the assignments that you've already turned in.