Wozer

assuming you do have joists/trusses up there you can either buy a stud finder and locate them, or tap around with your knuckles to find them..I advise the stud finder as even for a pro with good ears its hard to find them without using a punch to drive holes up there to make sure you get it right (and 'right' would be dead center in the structural member)...if the tape/float job is marginal you can probably look up there with good lighting and get a damn good idea of where they lie....

pre-drill holes for the threads of the hooks to make sure you don't start splitting any wood....make the bit size at least a 32nd smaller than the body of the hook (that would be the area inside of the threads)...I'd be tempted to go a 16th or 3/32 to make sure I got good solid bite with the threads...