Well one way that I do it, is I search out funnel clicks or solo ad groups on facebook. Then join the group, and start looking for the solo ad sellers with the most and best testimonials. Then I purchase smaller solo ads from these people to test their list out. So usually to test them I buy 100 clicks first, this usually gives me a good idea of how well my solo ad or funnel clicks will do with their list.
There are also many other ways to find reputable solo ad sellers, like the site Udimi. I am a member there, and I search in my members area for people with the best feedback to purchase from.
Otherwise you also have people who post there results and if they got sales from places like..
All of these places usually have reviews posted from solo ads bought, and whether or not they got sales.
Most solo ad providers will have packages available that are very small so you can test your ad copy before making a bigger purchase.
Hope that helps, and hope that is what you are asking about.
To your success