A quick and easy way to check to see if you are unstable approach is with the acronym C-FLAPS:
Flight path (to the runway), proper.
Landing configuration, set.
Airspeed, within normal approach criteria.
Power setting, adjusted and constant.
Sink rate, not abnormal.
It’s important that you are stabilized on approach to runway to ensure that your landing is safe to carry out. If a number of those criteria aren’t met then you could consider the approach to be unstable, prompting you to go around.