With RTO selected the speedbrake will deploy automatically during an RTO. It is the Captain's responsibility to insure the speedbrake deployed during the rejected T/O procedure. Basically, it's autothrottles - OFF, throttles - IDLE, thrust revers - DEPLOY, then check the speedbrake is deployed. There are a lot of other things that need to be done, like note airspeed at the time of reject, call the tower, run the RTO checklist, etc., etc., but above are the basics.