Sometimes it is clear when the imperfect is an habitual action.
Yo dibujaba los lunes por la mañana. I used to draw on Monday mornings.
And other times it is clear that it is a continuous action.
Yo dibujaba el lunes por la mañana. I was drawing Monday morning.
Yo hablaba cuando mi profesor me interrumpió. I was speaking when my professor interrupted me.
However, there are times when i think it is ambiguous.
Rosa hacía su tarea a las 9:00. Rosa [was doing] / [would do] her homework at 9:00.
Eduardo bailaba y su novia cantaba. Eduardo [was dancing] / [would dance] and his girlfriend [was singing] / [would sing].