You are correct - unless you were there and insured the ground was raked smooth before the pour, you never quite know if there could be thin spots/areas just due to uneven ground. I was sure nervous so I measured the concrete thickness as each hole was drilled for my lift. The concrete was not as thick as I had spec'd (9") but the thinnest was 6-1/2" and went up to 8-1/2". My salesperson, who has been selling/installing lifts for 40 years, assured me it was good to go.