Sounds like tracking is more what you need to establish.
I can tell you that if I am looking for a gym or fitness instructor, etc.. referrals from friends would probably be big, but I am definitely going onto google as well.
Good digital marketing people will help you track KPI's for the SEO or things you are doing; how much people coming to site (from where), how many at key page in your website sales funnel, how many convert to leads.
I like to have 1 'conversion' page on the website. Each service page or info page is designed to funnel the visitors to that 'conversion' page (usually the contact page). I then setup A/B testing on that page (example, with optimizely) and then test out a couple of offers / variations to see which converts most.
Of course you need to have analytics setup, call-tracking, etc.. to know the results.
You can measure the effectiveness of the SEO companies if you have good analytics and tracking setup.