The direction of rotation of a DC motor, as shown below, is in the clockwise direction.
What is direction of the magnetic field?
Right to left
Let's observe the left arm of the loop.
Since the loop is rotating clockwise, its left arm is moving vertically upwards. So, the thumb of your left hand will point upwards.
The middle finger points towards the direction of current, hence, it will be pointing along the left arm and away from you.
The forefinger now gives us the direction of the magnetic field, which is from right to left.
You could also verify this with the right arm of the loop as well. The right arm is moving downwards, and the current is directed towards you. So the forefinger will again point towards the left.
Hence, the magnetic field is from right to left.