In the Grade Change Activity log (accessed under Admin Tools > Logging), you can search by grader, student, course, and assignment. But if you try to search by multiple fields at once, it doesn't work. For example, if you try to search for all grades entered by the grader "Bob" for the student "Sue," it will just give you all the grades for Sue and ignore the "grader" field.
Instead, you should be able to fill out multiple fields at once, and Canvas will only return grade activity that matches all of them. This would really help with narrowing down to what you're looking for in a grade log with dozens or hundreds of entries.
Other Grade Change Activity log ideas: