The undercarriage has a breakable fuse around the edge of the wheel well. When it has broken, the landing gear leg will extend back down. You will know it has happened because the landing gear indication lights will read Red and Green, and there will be a reduced amount of Hydraulic A quantity.
Ref: FCOM2 / OMB.
Last edited by B737900er; 1st August 2017 at 10:12.