You should in all likelyhood not be using dd to begin with. But here's how to monitor progress if you do.

On Linux, to view the progress of the file/disk copy tool dd, you could send the USR1 signal to get a progress output. This apparently doesn’t work on Apple’s OS.

However, with Activity Monitor, it's easy to see the progress of dd when, for example, copying an operating system image onto a USB (which can take a while...). Simply compare the size of the image with the "bytes written" column to get a good idea of how much progress it has done:

If you need to view more detailed progress, or use dd lot, you can try installing pv, a utility which echoes the amount of data piped through it. One would use it with dd like this:

dd if=/file/image | pv | of=/dev/disk8

That would render something like this, letting you know the progress:

1,38MiB 0:00:08 [  144kiB/s]

Also, with pv, you could specify the --size parameter to get an estimation of the time it will take to finish. pv can be installed with, for example, Homebrew.