How can you tell the difference between sadness and depression?

It's harder to tell nowadays after being on lamotrigine for several months. I feel like nowadays that the amount of time is shorter (days) and can usually be interrupted by something such as long term distractions. However those days could feel soul crushing. But what happened to me recently to trigger this bout (one day now), I don't know if I'll recover from for a long time. At what point do I consider adding on an antidepressant? I don't know if I know what just sadness feels like.