Hiring is where most SEO budgets are won or lost, and the evaluation usually happens with the least information anyone will have about the relationship.
Ask what they would not do
A consultant who has never turned down work, never told a client a tactic was too risky, and has no examples of recommending against spending is either inexperienced or agreeable in a way that will cost you.
Guaranteed rankings are disqualifying
Nobody controls search results. A guarantee means either the target terms are worthless, or the method carries risk that will be discovered later. There is no third option, and it is the fastest single filter available.
Ask how they would measure failure
Anyone can describe success. A practitioner who has run real programmes can tell you what the first sign of a failing engagement looks like and when they would recommend stopping. That answer reveals more experience than any case study.
Check the reporting before the contract
Ask for a sample report. If it leads with rankings and impressions and never reaches revenue or leads, that is what you will receive every month, and the relationship will end in an argument about whether it worked.
Scope the first engagement small
An audit or a defined three-month project reveals how someone actually works. A twelve-month retainer signed on a pitch reveals nothing until it is expensive to exit.