As dd(1) doesn't display (unprompted) progress info, it's convenient to use
pv(1) for this purpose instead, e.g.:

    # pv <imagefile |sudo dd bs=2M of=/path/to/stick oflag=sync

The last part forcing synchronous output is important -- otherwise the pv(1)
will just fill the systems file system buffers and reach 100% immediately, and
then those buffers will slowly drain into dd(1). The drain status may be
gleaned from:

    # grep '^Dirty:' /proc/meminfo

.. but this is a total of all buffers, and so may not correspond directly to
the particular disk one happens to be interested in at that particular moment.