How can I track the progress of dd after I've already started it? I'm migrating my HDD to an external hard drive so that I can do a clean install of macOS Mojave.

I'm using dd if=/dev/disk02 of=/dev/disk15s2 to accomplish the task.

disk02 is the Macintosh HD (SATA-HDD), disk15s2 is the WD External HD

I didn't realize that it wouldn't show me any indication of progress (the HDD has ~750GB on it) - this is the first time I've done this through the terminal.app in recovery mode. 

Is there a way for me to track the progress in another terminal window while it is in progress?

I'm on a Macbook Pro mid-2012 running El Capitan.
Yes, from a separate terminal you can issue this command:

kill -INFO 1234


where you need to replace 1234 with the pid of your dd process. You can look it up with the ps command.

Another simplification at a second terminal:

sudo kill -s siginfo $(pgrep ^dd)   # get dd info


Another simpler way is to request the progress information from the same terminal as dd by pressing Ctrl-T.