How can seafloor spreading be detected?
Presently using GPS gives accurate readings of sea floor spreading
The modern use of Geological Positioning allows for accurate measurements of the position of the sea floor allowing the detection of seafloor spreading.
The first evidence of the seafloor spreading was the detection of alternating magnetic strips on either side of the mid Atlantic ridge.
The intensive use of submarine warfare and counter measures revealed the existence of the magnetic stripe
The concept that the earth was spreading was tested in the rift valleys. Tension rods were placed in the cracks created by the rift valley. As the rift valley spread the tension rods increased the length allowing the detection of the earth's spreading.
The theory is that the rift valleys will create new oceans, so the detection of spreading in the rift valley provided evidence of seafloor spreading.