Sweet potatoes are maturing when the stem becomes woody. The thickness of this stem can be a good indicator of how well the plant is producing.
It's natural to be curious.
If you must know what your plant has been up to, then you can dig gently around the plant to get an idea of size. Just make sure to cover them up again and let them get back to growing.
Sweet potatoes have a top and a bottom. This one grew very well, popped out of the soil, greened up and started to make slips right in the garden.
For this reason you should check on your taters late in the season to ensure that they stay covered.
This can be solved by slightly mounding with soil as with potatoes or consider adding a layer of mulch.