Obviously if the partition is perpendicular to the ceiling joists then find one using a metal detector to find the nails or simply push a screwdriver through along a line that will be covered by the head rail of the partition untill you find a joist, the next one will be about 400mm along.
If you are parallell to the ceiling joists and you are lucky, one may be right above where you want it, if not, make up your timber frame on the floor about 10mm shorter than the ceiling, lift it into place and secure with a few wedges, fill the 10mm gap with expanding foam along the entire length and if possible use screws to fix the frame to the walls at either end. Use plasterboard scres not nails to fix the boards, scrim tape all edges and whilst you may not be skimming the existing walls and ceiling, you should feather edge a narrow band to cover and secure the scrim.
foam and plasterboard may not be strong in localised fixing points but over large areas they are more than strong enough to hold. I have recently fitted windows into a rubble built house where it was impossible to find purchase for frame screws. between the foam all the way around, the outside render and the inside plaster they will withstand any winds that they are likely to see
