Andiamo is right, every trial is different. But to give an example, I had to go to the hospital on 2 separate days. They took blood and did all the routine measurements, also did a cardiogram. I had to answer the usual barrage of questions about my previous and current health and any meds I regularly took. They also gave me a liver biopsy as I had never had one.
You are better to read the inclusion / exclusion criteria first. In my trial at least they were pretty serious about eliminating anybody who didn't fit their bill,
dointime
Each trial is different, so you need to get a copy of the inclusion/exclusion documents. You can usually find that on the government website.
http://www.clinicaltrials.gov/
Good luck!
Eric