As long as they have a clear presence in the market (IE, a website that suggests they have been promoting similar offers for a while), I give access to the product for free. Almost no one worth their salt will promote something without checking it out first, so it goes with the territory. You just need to ascertain whether or not they are legit. I think it's usually pretty obvious. There aren't many people that would put together a fake website, or even a social media profile, just to get a free course. And if they were full of it, they never would have bought anyway. I think you're especially safe in your market and I wouldn't hesitate to give free access to affiliates that feel even remotely legit.

You can check with quantcast and Alexa but that is really only helpful for very big sites. I wouldn't seat it too much. If they have any presence at all they are probably legit. I've had a few people ask for copies with no web site that I haven't given copies to, but I'm yet to feel that I was ripped off by someone only looking for a free copy. I would just use common sense and try to be generous where it makes sense. A good affiliate is very valuable. 

There are people who are pretty strict with review copies but I'm not one of them.