Nobody gets paid for their blood donation in Canada. Nobody. At best, they'll give you some orange juice and a couple of cookies afterward, but the voluntary aspect of donating helps to eliminate individuals who donate just so they can make a few bucks that they'll use to hook up with their dealer afterward.
They verify that your hemoglobin level is adequate for donation during the intake process, then send you to the nurse to check your blood pressure and pulse. The nurses also examine your antecubital fossae for track marks.
One last thing that happens after you go through the "highly personal" questions with the nurse. You are given a ballot that has two stickers on it. One sticker says DO the other says DO NOT. You are then asked to fill in the blank space on the ballot with one of the stickers in the line that reads: Please ______ use my donation. This helps people to "save face" if they are part of a group that comes in to donate blood, but know deep down that they have engaged in risky behaviours that would jeopardize the safety of the blood supply. (Some employers will give a half-day off with pay to donate blood, if you attend the donation centre as a group.)
I'd rather go through this every single time I donate rather than let one questionable unit of blood enter the system.