Visually checking both spoilers as part of the pre-landing check was drilled into me when I was being trained in the 1980s and I have never landed a glider since then without doing this. The reason is not only to check that you have the proper handle but also that they deploy symmetrically. In unflapped ships, I was also taught to keep my hand on the dive brake lever until safely on the ground.

I have witnessed two cases of mistaken handles - one in which a flapped two-seater flew the entire length of the strip with its flaps waggling instead of dive brakes.

Mike