Some of the problems I run into when investigating a program:
1. Reviews
Doing a search for any reviews that are "real" reviews is frustrating. Most of the reviews are negative reviews from someone in Wealthy Affiliate trying to get you to join. You usually have to dig pretty far into the pages of Google to even find a real review.
2. Scam Report Sites
A lot of them are from people who didn't follow directions and didn't get paid or something similar. Some of these are "click bait" type postings that really don't tell you much of anything.
3. MLM or Pyramid (Ponzi) scam?
Usually most of these you don't really know for sure until you join. From the ads it is hard to tell on a lot of them which is probably on purpose. Even then most people don't know the difference and some of the owners don't even know they are doing something illegal until they are shut down.
4. You really don't know until you join
There are a lot of programs that seem legit and you can't find anything online about them if they are a fairly new program. You just don't know until you join to find out the "real deal". Then you find out the great training is PLR reports from 10 years ago, a forum or Facebook mastermind group nobody has posted anything on for years, or other bad signs. All of these are the "exclusive" things you won't get access to until you actually join. this includes how you are paid. I have been burned from a few that seemed really good until I joined and found out you didn't get paid until you had X number in your downline, a ridiculously high payout minimum unless you upgraded to a much higher level, you join free but have to actually spend money to have any chance of making money, or some other weird requirement. I have joined some that even after joining I was totally confused by the complex methods they used to determine what you were paid; the "matrix" was just too complex and confusing.
The ads you see are designed to entice you to join or plunk down your cash. Even with a lot of experience with "reading between the lines" in some ads, you will still get burned. Of course even in the worst programs imaginable, there are people who swear by them even if they haven't made a dime and have little to no chance to either - like the lottery.
Bottom Line:
You just have to use some common sense and understand how a business really works. If they are telling you something that flies in the face of what you know about how a business works, then it is probably something to avoid. Sometimes simple math will tell you the true story, like these "pay $2 once (or even $0.50, $1, or $2 a month) and make thousands" just takes some simple math to understand they are full of it. You have to understand how money is really made and not some fantasy idea. If you think the money just happily and magically appears brought by the unicorn caravan, you have some serious issues yourself. 
