Unfortunately I don't believe that you can in the replay.
However, the data can be mined from a combination of sources.
You can look at the unit window at the end of the game and see how many harvesters they have and then compare it to the number of workers created at the score summary.
This only helps if you have the game still in your recent game history in which you can see the score screen which you should if you are looking at a game you just played.
Edit: Your other option is to watch the income overlay along the left hand side during the course of the replay. It shows you the number of harvesters at that moment in time so as the game goes on you can see how many they currently have. You'll still need to do math to see what they started at and what they've fallen to after an attack to get "Workers Killed" in a skirmish, but you won't have a running tally unless you go with the good old pen/paper method along with that.