Using an analytics software (Google Analytics, annotated with SEO initiatives), and a rank tracker such as White Spark, individual keywords being tracked from several zip codes surrounding the dealership, is an effective way to measure increases in organic traffic and local search results. Your service provider will also have access to industry software such as Moz, ahrefs, and/or SEMRush, to name a few.
Good article, but run from any package offered at a $500 pricing point. "Any one local SEO initiative will likely realize diminishing returns". Not to be cynical, but you can see the cost within this article necessary to realize a local business' organic potential - SEO spends at this level unfortunately is largely throwing money out the window.
A good but overlooked way to evaluate a service provider is reading their Glass Door reviews. If you sense internal chaos this will certainly be reflected in the quality of work and ROI on your SEO investment.
