Follow the steps below from the Zhu Badminton Centre to determine if your shoes are non-marking, and therefore suitable for use indoors.
Place the shoe so the sole is facing up and press your fingernail into the sole. If there is an indentation left behind the sole is likely made of a ‘soft’ rubber and is non-marking.
Place a piece of plain white paper on the floor and, holding the shoe, firmly rub the sole across the paper, being careful not to tear it. If you observe a scuff mark on the paper then you do not have non-marking shoes.
Wearing the shoe this time, drag the heel across another piece of white paper. If there are no markings visible on the paper, the shoes are non-marking. Note that non-marking shoes may also have dark-coloured soles.
Make sure to clean your shoes before testing them, as built-up dirt and debris may leave behind scuff marks on the paper irrespective of whether or not they are non-marking. Regularly cleaning your indoor court shoes will also help ensure you have the best grip possible and are less likely to slip and injure yourself when playing on smooth indoor surfaces.