While there is no dedicated 'symmetry line' sketch tool, you can try:
1. Use construction lines to mark symmetry. These will be listed as curves in the drawing sheet only and not visible in the source model.
The drawback is they will not relocate when views are moved unless you group select the curves and the view when moving them.
2. Use the built-in symbols from the note and dimension annotation tools.
3. Annotate with the GD&T symmetry symbol (defined by ASME Y14.5 standard) and connect to the view using a note leader. Using this method means the GD&T symbol always moves with the view.
Please take a look at the GIF for a demo.