For me, there are a couple of primary ways. The main one is a proven record of results. When someone inquires about my services, I refer them to past and current clients. 

Because one of my key approaches is How to Differentiate Yourself to Eliminate Your Competition, they have no fear or reluctance to discuss their success. It is a very effective and passive way to validate who you are and how you work.

The second big way is content. I write and publish frequently. Unlike many others, I do not publish "useful but incomplete information." The old approach was to tease them by creating interest, but not telling the full story. People do not have time for that anymore. 

I give the complete strategy. My approach is that it will take you X months or years to implement this on your own. If you want to cut that time to 1/4 or less, we should talk. Having an expert guide dramatically speeds up the learning and implementation process. In my case, clients typically achieve in 6 months what previously took 2-3 years, with lower results.