The “Hyper Suprime-cam” instrument came online in 2012, so it’s not new, and was presumably calibrated against well-understood targets; so its light-detection capabilities have been well tested. The physics of gravitational lensing is also well understood and verified, so they know what light they’re looking for.

If you can test each part of a system independently, and the parts connect in a reasonably trivial fashion, it’s about as good as testing the whole.