Print out this pdf file on an A4 piece of paper (it should cover it without resizing - pdf is good that way). Have the page 6 metres away from your eyes in good lighting, and check your vision with one eye covered at a time.
People with good visual acuity ("20/20 vision") should be able tell the direction of the C shapes. Anyone who cannot see down to the bottom lines of the chart properly should go to an ophthalmologist for proper testing and investigation.
This chart is gratefully acknowledged to be copyright Victoria Francis, 1996.