You are unlikely to be able to assess the technical quality of the work before buying it. That is fine, because the things that predict whether an engagement goes well are mostly commercial rather than technical, and you can check every one of them in a first conversation.
Ask what happens if you leave
The answer tells you almost everything. If the ad accounts, website, and analytics are in the agency's name, leaving means starting from zero: losing years of conversion history that makes campaigns efficient, and rebuilding a site you thought you owned.
Ownership should be yours from the first day, not offered as a concession when you try to leave. Ask directly, and ask for it in writing.
Ask how they are paid
An agency charging a percentage of ad spend earns more when your budget rises, regardless of what the budget produces. It does not make people dishonest, but it does make one recommendation permanently easier than every other.
The test is simple. Ask what they would do if the data showed you should halve your spend. Under percentage pricing that advice costs them money. Under a flat fee it costs them nothing.
Ask what the report opens with
Impressions, reach, and rankings are diagnostics. They tell you whether the machinery is running, not whether it produced anything. A report that leads with them is a report designed to look busy.
The first number should be enquiries, or better, enquiries that became work. If an agency cannot report that, either the tracking does not exist or the numbers are not flattering. Both are worth knowing before you sign.
Ask who actually does the work
The pattern to watch for is a senior team in the pitch and a junior or outsourced team in delivery. Ask who will be in your account weekly, what else they work on, and whether any of it is subcontracted.
Ask about your competitors
Most agencies will happily run marketing for two competing businesses in the same city. Everything they learn from your budget becomes available to the company you are trying to beat.
Exclusivity is rare because it is expensive to offer, which is precisely why it is worth asking about. If an agency will not commit to a market and industry, assume they intend to sell the same playbook next door.
Watch for guarantees
Nobody can guarantee rankings, leads, or revenue. Too much sits outside any agency's control: competitors, platform changes, seasonality, your own pricing and follow-up. An agency promising a specific outcome is either inexperienced or counting on you not reading the qualifications.
What can honestly be committed to is process, transparency, terms, and pace. Those are the promises worth extracting.
Then ask what they would not sell you
The most useful question in any agency conversation. A good answer names something specific and explains why it would be premature for your business. An agency for whom the answer is always "all of it" is describing its pricing, not your plan.