There's not way to know whether your writer signed or not through interfolio. You have to ask the writer personally.
Officially letter head is required at 99% of ther schools. It just means that the lor must be printed on offical university paper or offical clinic or hospital papers. ( with the logos and address/department/ blah blah on somewhere of the paper) I believe this process adds credibility, since it's hard for students to obtain or steal such papers!