I had a tilt test done.
They place you on a type of bed, and strap you on to it, and leave you lying down for maybe 10 minutes. Then they raise the bed to standing position. It is possible that you may pass out at that moment since your pulse and heart pressure change, but I didn't. Then they leave you in that position for about half an hour, and you may pass out then because you have been in that one standing position for too long, and your heart rate and pressure change. I passed out when I was standing, but it is also possible that you will not pass out at all.
It is not very scary, I am just 13 years old, and I thought it was ok.
I haven't had a tilt test done but found this information for you.
http://www.heartsite.com/html/tilt_test.html