ok i just looked at Jetstar website and see it offers online check in
but even if i knew this had to be done separately (the QF site came up with all details of my booking and just said 'no online checkin for JQ' - It could have more usefully said 'go to JQ website for online check in for this flight') i only had the QF booking number for the complete return booking, i had never received a JQ PNR