AC/DC coupling and DC offset to read DC voltage in analog oscilloscope
You can short circuit the input to ground, this gives you a straight line at 0 V. You can position this with the offset knob at a position you like - this will be your zero volt position.

Now you can switch the input from ground to a DC coupled input. You will see your waveform, the AC as well as the DC. The DC content is the offset of the middle of your curve to the before positioned zero line.

See the red dashed line in this picture: Suppose you configured this as your zero line. Than the distance assigned with the arrow is your DC offset.