some schools screen for secondaries, others give them to everyone, and some don't have them.
schools that screen (e.g. ucla, ucsf, etc.) will send you an e-mail if you make the cut after they review your application. you're supposed to complete and send these back within a couple weeks and some have strict deadlines.
schools that don't screen (e.g. penn, etc.) will have them available on their website. check there once you submit your aadsas.
some schools either don't have secondaries (e.g. columbia) or just send them out as questionnaires (e.g. usc) once you're selected for an interview. usually these get turned in on or before your interview date.
i'm just throwing these schools out there from memory as examples...could be wrong so don't take my word for it.
basically you'll have to be on top of things on a school-by-school basis once you submit your aadsas primary applcation. it varies.