Hi, It’s a little bit tricky in these days as some are trying to sell something that they have no proof of working, just because they read it or heard it from someone else.
As an SEO expert myself I believe in getting the homework done before trying to sell something or offer a free trial, the first month free results in advance, or something…
It’s hard to pick the right and trusted SEO expert as it is hard for an expert to choose the right client.
I don’t think that there is an exact formula on how to choose the best SEO expert the first time. But asking questions and carefully listen to the answers is a good start.